Car Radio Wiring Diagram

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A car radio wiring diagram identifies the standardised ISO 10487 connector pins for constant 12 V, switched 12 V, ground, illumination, and speaker pairs — enabling head unit replacement without cutting factory wiring.

The car radio wiring diagram centres on the ISO 10487 standard, which defines the two 8-pin connectors used to connect an aftermarket head unit to the vehicle's factory wiring harness. This standard was developed to allow head unit replacement without permanently modifying the vehicle's wiring, using a vehicle-specific ISO adapter harness that maps the car manufacturer's connector to the standard ISO pinout.

The ISO 10487 standard defines two connectors, conventionally called connector A (power and system functions) and connector B (speakers).

Connector A pins: - A1: Speed signal (vehicle speed sensor output for navigation-equipped head units) - A2: Not used in many vehicles - A3: Reverse trigger signal (12 V when reverse gear is selected) - A4: Mute signal from telephone or hands-free - A5: Ground (chassis earth, also marked as GND or Battery −) - A6: Battery positive 12 V — constant, always live regardless of ignition position (marked B+ or Battery +, typically yellow wire in aftermarket head units) - A7: Ignition-switched (accessory) 12 V — live only when ignition is in ACC or ON position (red wire in aftermarket head units) - A8: Illumination positive — live when the vehicle's instrument panel lights are on, used to switch the head unit's display to dimmed/night mode

Connector B pins (speakers — four pairs): - B1: Right front speaker positive (RP+) - B2: Right front speaker negative (RP−) - B3: Left front speaker positive (LP+) - B4: Left front speaker negative (LP−) - B5: Right rear speaker positive (RP+) - B6: Right rear speaker negative (RP−) - B7: Left rear speaker positive (LP+) - B8: Left rear speaker negative (LP−)

Factory speaker wiring may not use a separate speaker negative conductor connected to chassis ground — many manufacturers use a floating speaker output from the factory amplifier where neither speaker terminal is at chassis ground potential. Connecting such a speaker output to an aftermarket head unit's speaker output without understanding this can cause damage. Always use the appropriate vehicle ISO adapter harness rather than hard-wiring into the factory connector.

How to wire car radio wiring diagram

Identify the vehicle's ISO connector type and required adapter harness Determine the vehicle make, model, and year. Look up the required ISO adapter harness for that vehicle. Most adapter harnesses include colour-coded wires pre-labelled with their ISO function. A vehicle-specific adapter harness is the correct approach for any vehicle — do not splice directly into the factory connector.
Disconnect the battery negative before starting Remove the battery negative terminal. Many modern vehicles have airbag control modules near the dashboard — disconnecting the battery and waiting at least 10 minutes (check the vehicle service manual for the specific wait time) before working near the dash reduces airbag deployment risk.
Remove the factory head unit and identify the factory connectors Remove the factory head unit using appropriate trim removal tools. Identify the factory wiring connector(s). Plug the vehicle-specific ISO adapter harness into the factory connector. Do not cut or modify the factory connector.
Connect the ISO harness to the aftermarket head unit The adapter harness typically terminates in colour-coded wires or a standard ISO connector. Match colours and functions: yellow to yellow (constant 12 V), red to red (switched 12 V), black to black (ground), speaker pairs matching colour codes. If the harness terminates in bare wires, use insulated butt connectors or a certified wiring connector — do not use tape alone on bare wire joints.
Connect the antenna adapter and any auxiliary wiring Connect the vehicle's antenna lead to the head unit antenna input via an appropriate adapter if required. If the vehicle uses a powered antenna, connect the head unit's powered antenna or mast motor wire (typically blue or blue/white striped) to the adapter harness's antenna control output.
Test all functions before refitting the head unit Reconnect the battery and test: head unit powers on with ignition; head unit retains memory when ignition is off; all four speakers produce sound with correct polarity (a portable speaker polarity tester makes this quick); illumination wire dims the display when headlights are on; any reverse camera trigger functions correctly.
Refit the head unit and dashboard panels Tuck all wiring neatly behind the head unit, ensuring no wires are pinched between the head unit chassis and the dashboard aperture. Slide the head unit into its mounting sleeve and secure. Refit all trim panels in reverse of removal order.

Specifications

ISO 10487 Connector A, Pin A5	Ground (chassis earth, −12 V)
ISO 10487 Connector A, Pin A6	Battery positive (+12 V constant, memory) — yellow in aftermarket head units
ISO 10487 Connector A, Pin A7	Ignition switched ACC/ON (+12 V switched) — red in aftermarket head units
ISO 10487 Connector A, Pin A8	Illumination positive (instrument panel lighting)
ISO 10487 Connector B, Pins B1–B8	Four speaker pairs: right front +/−, left front +/−, right rear +/−, left rear +/−
Aftermarket head unit ground wire colour	Black
Aftermarket head unit remote antenna/amplifier wire colour	Blue or blue/white striped
Standard speaker impedance (factory and aftermarket)	4 ohm nominal

Safety warnings

Disconnect the battery negative terminal and wait at least 10 minutes before working in the dashboard area. Modern vehicles have airbag squib (inflator) circuits routed through the dashboard — these circuits retain capacitive energy after battery disconnection and can fire the airbag if disturbed while energised. Consult the vehicle service manual for the specific discharge interval.
Never cut or permanently modify the vehicle's factory wiring harness if it can be avoided. Use the appropriate adapter harness. If a fault develops later, an unmodified factory harness is far easier to diagnose and restore.
Verify the polarity of all power connections before reconnecting the battery. Reversed polarity on the constant 12 V or switched 12 V connections will immediately damage the head unit's internal electronics.
Do not exceed the head unit's rated speaker output impedance. Connecting 2-ohm speaker loads to a head unit rated for 4-ohm minimum will overheat and damage the internal amplifier.

Tools needed

Trim removal tools (plastic pry levers — do not use screwdrivers near interior trim)
Multimeter (DC voltage for verifying circuit identity before connecting)
Wire stripper and crimping tool
Insulated screwdrivers
Speaker polarity tester (optional but recommended for confirming speaker phase)
Torx and Philips screwdriver set (head unit mounting screws vary by vehicle)

Common mistakes

Connecting the constant 12 V (yellow, memory) wire to an ignition-switched source — the head unit loses its clock and presets every time the engine is switched off.
Connecting the switched 12 V (red, ignition) wire to a permanently live source — the head unit remains powered permanently, draining the battery and preventing theft deterrent from functioning.
Not using an adapter harness and cutting the factory connector — permanently modifying the vehicle loom and making future restoration or fault diagnosis more difficult.
Ignoring that a premium factory audio system uses a separate amplifier — installing an aftermarket head unit into the speaker outputs of a factory amplified system produces barely audible sound.
Connecting speaker wires with inconsistent polarity across different speaker pairs, causing acoustic cancellation and degraded stereo imaging.
Skipping the test phase before refitting trim panels, requiring complete disassembly again if any function does not work correctly.

Troubleshooting

Head unit turns on but immediately switches off or restarts in a loop: Cause: Insufficient current supply on the switched 12 V (red) wire, or the switched and constant wires are swapped Fix: Measure voltage on the red (switched) wire with ignition on — should be 12 V. Check that the yellow (constant) wire is on the permanently live source and red on the ignition-switched source. Verify the ground connection has low resistance.
No sound from one or more speakers: Cause: Speaker wire not connected, open circuit in speaker wire, or speaker itself faulty Fix: Verify the speaker wire connection at the head unit or adapter harness. Test continuity from head unit output to speaker terminals. Confirm the speaker is functional by briefly connecting a known-good speaker to the head unit output.
Display does not dim when headlights are switched on: Cause: Illumination wire (A8) not connected or connected to the wrong circuit Fix: Verify the illumination wire is connected and that the wire at the adapter harness side reads approximately 12 V when the vehicle's instrument lighting is switched on. Check whether the vehicle uses a PWM dimming signal on this wire — some head units support this, others do not.
Weak or distant sound from all speakers after installing a premium system head unit into a vehicle with a factory amplifier: Cause: The vehicle has a factory amplified audio system; the head unit is connected to speaker-level inputs designed for amplified output, not the line-level output feeding the factory amplifier Fix: Research the vehicle audio system to confirm whether a factory amplifier is fitted. If it is, either use a head unit with a factory amplifier retention interface (keeps the factory amp and provides digital control), or bypass the factory amplifier and connect the aftermarket head unit directly to the speakers.

Frequently asked questions

What is the difference between the constant 12 V (yellow) and switched 12 V (red) wires?

The constant 12 V (A6, yellow in aftermarket head units) is permanently connected to the battery and powers the head unit's memory — storing presets, clock, and settings. It must never be disconnected when the vehicle is parked. The switched 12 V (A7, red) is live only when the ignition is in ACC or ON, and it is this supply that actually powers the head unit on and off with the ignition.

Why does my replacement head unit lose its presets every time the car is switched off?

The constant 12 V (memory/battery) wire is not connected, or is connected to an ignition-switched source rather than a permanent supply. When the ignition is off, the head unit loses power entirely and cannot retain its memory. Verify that the yellow memory wire is connected to a permanently live 12 V source, typically through the vehicle ISO adapter harness.

Do I need a vehicle-specific ISO adapter harness?

In nearly all cases, yes. The vehicle manufacturer's factory connector uses a proprietary multi-pin connector that does not physically match the ISO 10487 standard. A vehicle-specific adapter harness converts between the two, typically with colour-coded wires that match the aftermarket head unit colours. This avoids cutting the factory loom, preserving the ability to restore the original head unit.

What happens if I connect the speaker wires with incorrect polarity?

Reversed polarity on one speaker causes that speaker's cone to move in the opposite direction relative to all others. In mono or adjacent-speaker configurations, this causes acoustic cancellation in the frequency band shared between the reversed speaker and its correctly wired counterpart. Bass reproduction is typically most affected. Always connect + to + and − to − consistently across all four speakers.

Can I use the factory speakers with an aftermarket head unit that has built-in amplification?

Yes, in most cases. Most factory speakers are 4-ohm loads compatible with aftermarket head units. The concern is factory vehicles with premium sound systems that use a separate factory amplifier between the head unit and speakers — in these systems the head unit output is at line level (very low voltage), not speaker level. Installing an aftermarket head unit directly into this wiring will produce very low volume. Identify whether the vehicle has an amplified factory system before the installation.

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