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What Is Accessory Wire on Car Stereo

The 12V accessory wire is a power wire in the car stereo wiring harness. Also called ignition or switched 12V wire, its sole purpose is to provide 12V power to the head unit when the ignition key is in the ‘accessory’ position.

How to Identify the 12V Accessory Wire?

All major aftermarket car radio brands (Pioneer, Alpine, JVC, etc.) color the accessory wire red for their head units.

But the problem arises when we’re dealing with the OEM stereos. Because in these stereos, the accessory wire can be colored anything from red and blue to pink and purple.

Go through the owner’s manual and check the wiring diagram to understand the color scheme properly.

Even if you’ve lost the manual and can’t find any information online, there’s still one last method to identify the accessory wire correctly.

For that, we’re going to need a multimeter. Here’s how it goes:

  • First, identify the ground wire. This wire is colored solid black in both aftermarket and OEM stereos, making it easier to recognize.
  •  Turn on the ignition key.
  •  Connect the negative multimeter lead to the ground wire and the positive multimeter lead of each car stereo wire one by one.
  •  Whichever wire gives you a 12V reading, there will be two possibilities. It’ll either be the constant 12V wire or the accessory wire.
  •  While multimeter leads are connected, turn off the ignition key.
  •  If the reading goes to 0V with the ignition key off, that wire is an accessory wire. Otherwise, it’s the constant 12V wire.

Types of Car Stereo Power Wires

There are different types of car stereo power wires:

  • Constant 12V Wire – This yellow-colored wire is directly linked to the battery and provides constant 12V power to the head unit, regardless of the ignition key position. But the sole purpose of this wire is to keep track of the ‘memory’ functions of the head unit, such as current date/time, radio station presets, equalizer settings, etc., and not turning on the radio.
  •  Accessory Wire – This wire is the one that turns on the car radio when the ignition key is in the accessory position. Without this wire, the radio would stay on/off all the time.
  •  Ground Wire – In addition to these two, there’s a -12V ground wire in all car stereo harnesses. Like the amplifier’s ground wire, this one allows the current to return to the battery. Thus, completing the ‘path.’ Without this wire, you’ll get different types of electric noises in your sound.
  •  Dimmer wire – This one is not as common as the other three wires we’ve discussed, nor it’s as critical for the overall car radio’s functionality. Its primary purpose is to adjust the car radio’s display and controls’ brightness depending on your vehicle’s interior lighting. It’s usually colored orange with white stripes.

Where to Connect ACC Wire?

If you have a wiring harness, you don’t need to do anything except plug the stereo harness into the dash harness.

If you don’t have a wiring harness, you’ll need to connect the car’s ACC wire to the stereo’s ACC wire. But this method is not perfect, as you’ll have to cut these connections in case of future upgrades.

What Happens If You Don’t Connect the ACC Wire Correctly?

There are two common situations in this regard:

  • You didn’t make any ACC wire connection at all. In that case, the car radio would not turn at all. That’s because it’s not getting any power through this wire.
  • You accidentally damaged the ACC wire’s pins on the wiring harness and connected the ACC wire and constant 12V wire together in the latter’s slot. In that case, you’ll be able to turn on the car radio. But it would stay on all the time (since it’s now linked to the battery) and eventually drain it.


By now, you must have a good idea about what an accessory wire is. And how important it is for car radio installation.

Therefore, you should pay close attention to it whenever making any radio installation/diagnosis, as it can damage the head unit otherwise. If you don’t have access to the wiring diagram, you can identify it with the help of a multimeter, as discussed above.

Frequently Asked Questions

What is a 12V ignition wire?

It is a power wire that turns on the radio when the ignition key is in the accessory position.

Where Do I Connect The ACC Wire?

There’s no need to do anything if you have a wiring harness. Plug that harness into the dash harness of the vehicle you’re installing the head unit, and it will do the job.

What color is the 12V accessory wire?

In all major aftermarket head unit brands, it’s colored red. For OEM stereos, you’ll have to check the wiring diagram.

Which wire is the 12V constant in car radio?

It’s the yellow wire in 3rd-party car radios.

Can A car radio drain battery If the vehicle is in ACC mode?

Yes, a vehicle uses the battery’s power to turn on electrical systems (including the car radio) when the engine is not running. And since the alternator is not recharging the battery at that time, the former will start to lose power until it’s fully drained.

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John is an automotive enthusiast who has been passionate about cars and car audio systems for over 25 years.

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