Learning, Wiring,

How to Wire a Stereo to an Ignition Switch (Steps & Testing)

If you want to add a stereo system to your car, you’ll need to know how to wire it up.

The ignition switch wire extends from the car’s ignition system to its stereo. It provides power (~12 volts) when the ignition is turned on, and, unlike the constant power wire, it does not when the ignition is turned off, nor does it dim like the dimmer wire. It is usually in red and maybe labeled as ‘Accessory,’ but not necessarily.

Once you’ve identified the ignition switch wire, you can connect all the other wires to add the stereo system to your car. This guide will show you how to do it and connect it tothe ignition switch, which might be marked as ‘ACC.’

A Car Stereo’s Wiring

Not all car stereos are wired using the same color coding on their wires, but a typical scheme is shown below.

To know which wire goes where you must consult your car’s manual for the stereo’s wiring diagram or do some testing to identify the wires.

The Ignition Switch Wire

The ignition wire is usually red; if the wires are labeled, it might be marked ‘Accessory’ instead.

It’s the wire that only has power when the ignition is on, and it remains at a constant 12 volts, approximately. In the sample wiring diagram below, you can see that the ignition wire is the red wire connected to the terminal marked with the number 12. Yours may or may not differ.

car radio ISO connector wiring diagram
Sample wiring diagram for a car stereo

ignition wire labeled
Video | Manny Kyro

Testing for the Ignition and Other Wires

To be sure that a wire is the ignition wire and not another wire, we will have to distinguish it from other wires that also show 12 volts when tested with a voltmeter or multimeter. These other wires are the constant wire and the dimmer wire:

  • The constant wire retains power even while the ignition is turned off.
  • The voltage across the dimmer wire and ground is initially 12 volts (approximately) when the car is turned on, and the headlights are at full beam, but it reduces when the lights are dimmed.

If you rule out the constant power wire and the dimmer wire, the remaining wire with around 12 volts, which remains at 12 volts when the car’s ignition wire is turned on, and the headlights are dimmed, will be the ignition wire.

All other wires are usually speaker wires connecting to different car speakers. They are usually multicolored.

Wiring the Stereo System

Firstly, while wiring the stereo system, turn off the car and make sure the constant power wire (usually the yellow one) does not touch any metal.

Secondly, wire according to your car’s wiring diagram for the stereo or as per the testing you did above to find out which wire is which.

Joining Wires

If you need to join wires (of the same type, of course, though the colors may differ), you can wind the two ends together, but it will be better to use a soldering iron.

After soldering the wires together, cover them with a rubber coating or wrap insulation tape around the joint. Do the same for all other wires that need to be joined.

Final Voltage Checking

Before connecting the wires to the stereo, you might prefer to check them to ensure the stereo is getting the power it needs.

Do this using a voltmeter or multimeter:

  • The constant power wire will have around 12 volts all the time, whether the ignition is on or off.
  • The dimmer wire will have around 12 volts when the ignition is on but less if the lights are dimmed.
  • The ignition wire will have around 12 volts when the ignition is on, will remain at this level even if the lights are dimmed, and will have no power when the ignition is turned off.

Type of WireMultimeter ReadingColor (Normally)
Ignition (accessory)12 voltsRed
Power12 volts (constant)Yellow
Ground0 voltsBlack
DimmerVariable (0-12 volts)Red or orange

Wiring at the Ignition Switch

ignition switch terminal diagram

At the other ignition-switch end, you need to check or connect the ‘accessory wire.’

It might be red or purple, but not necessarily. If labeled, it might be labeled as either ‘stereo,’ ‘accessory,’ or ‘ACC.’ If not, one way to find out (other than the wiring diagram) is to trace the wire from the stereo to see where it goes.

accessory wire at the ignition-switch end
Video | MrOlvidado89

The ignition switch will have at least the following four terminals:

  • BATT (battery), usually in red
  • ACC (accessory), usually in red or purple ← , goes to the stereo.
  • ST (start), usually in yellow or brown
  • IGN (ignition), usually in red or yellow

Remember: Note the function of the wire, not necessarily the color, as it can differ from what is shown here.

Other Questions You Might Have

Does the ignition switch need to be grounded?

No. You don’t need to ground the ignition switch.


If the wires are not labeled, I would advise you to label them to make them easy to work with next time.

Video References

Quality Mobile Video

Manny Kyro

Carpar Automotive


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About Sam Orlovsky

AvatarCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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