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What Is the Illumination Wire on a Car Stereo?

The wiring in your car stereo is not always the easiest thing to understand since the wires have different types and colors.

One of the wires that can be confused with others is the illumination wire. So, what does the illumination wire do on a car radio? Let’s find out.

Generally speaking, the illumination wire is one of the wires coming out of the stereo harness and is used to dim the radio’s screen and light the radio controls. The illumination wire is connected directly to your car’s lighting system, and it sends a direct signal to the stereo as soon you turn the headlights on or off.

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Tip: While working with the radio wiring, you can rely on the colors of the wires, but it is handy to have a good voltmeter and make sure all cables are connected to the right places.

The multimeter that I use a lot, and I am happy with its quality, is the AstroAI TRMS 6000. It is easy to use, and it makes precise measurements of all electric parameters. 

In the article below, I will show you what the illumination wire is, how it works, and how to connect it with the radio and car installation.

What Does the Illumination Wire Do on a Car Stereo?

The illumination wire is used to transfer the signal from your headlight switch through the wiring harness to the illumination wire attached to the car stereo.

This is especially important when driving at night because you do not want the light from the screen to be too bright, which is a safety hazard when you’re driving at night!

When the headlight switch is turned on, it will switch power to the car stereo illumination wire, activating an internal circuit that controls how much light is shone from the display.

On the other hand, turning off the headlights will cut power from this wire and keep the screen at its whole light.

Note that some radios have a separate illumination wire for day and night mode. When the sun goes down, power is transferred from day mode to night mode so that you can see your radio clearly while driving at nighttime.

How to Connect Illumination Wire to the Car Stereo?

If you install a new car stereo into your vehicle that does not have the illumination wire attached, you need to know how to connect it yourself.

The few steps below will show you how to add an illumination wire to the stereo:

  • Disconnect the negative battery terminal under the hood using the socket wrench and the correct insert.
  • Locate the orange wire at the back of the stereo’s wire harness. In some radios, it can be an orange wire with a white stripe. To make sure you have the correct wire, check the color code with the radio manual or the label at the top of the radio.
  • Find the matching wire on the wire harness adapter that interfaces with the factory harness in the dash cavity. Look closely at the labeling on the wire harness adapter to ascertain which wire is the illumination lead.

To make sure you have the correct wire, you can make a simple test using a voltmeter or a bulb.

Turn parking lights on, and connect one probe to the tested wire and another probe to the car chassis. If you see on the display 12V after touching a probe to the wire, this is the wire you are looking for.

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  • If you found the corresponding wire in the car installation, the next step is to strip 0.5 to 1 inch from the flying ends of both illumination wires.
  • Insert the wires into opposite ends of a 16-gauge crimp connector and crimp it using the crimping tool. Squeeze firmly to secure the wires into the connector in the same way as for all other cables you connect.
  • Reconnect the negative battery terminal after all harness wiring is complete.
  • Turn on the radio and test by turning the headlights on. A correctly connected radio should dim with the lights on, then return to normal brightness when off.

What Is the Voltage on the Car Radio Illumination Wire?

The voltage on the illumination wire is 12 volts, which is provided by the car’s alternator while the engine is running.

Through the illumination wire, your car stereo will receive a constant signal as soon as you turn the lights on, and this allows for an automatic dimming of the display screen.

Is the Illumination Wire Necessary in Car Radio?

The car’s illumination wire is an essential part of the stereo because it provides a constant signal to your radio’s screen.

Without this wire, your car radio cannot receive a signal while the lights are on, which means that you will not be able to dim the radio screen after sunset.

What is the difference between illumination wire and dimmer wire?

The Illumination wire puts out 12V anytime your headlights are on, and the dimmer wire puts out between 0 and +12V depending on the brightness you have the headlights turned to.

Dimmer wire allows the radio to dim with the rest of the indicator lights. Many older cars do not have it, but these days, you will find a dimmer wire in most cars.

Because most of the aftermarket radios are either completely dim or completely bright, it doesn’t really make a difference to which wire you will connect the orange one from the radio harness.

Dimming wire is especially useful if you have a large screen head unit that can be really bright at night.

If you have the dimmer wire connected, as soon as you turn on your headlights, the radio dims, and you don’t have a bright monitor shining right at you when you’re driving.


The illumination wire will tell the car stereo when to turn on and off. When a car’s headlights are turned on, a signal is sent from the headlight switch through the wiring harness to the illumination wire attached to the car stereo.

This is important when driving at night, and you do not want the light from the screen to be too bright.