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Hi all! I hope this is the right place to post. I've noticed while researching this topic for myself that there is still a lot of uncertainty as to how to wire up the headlights to dim your new car stereo. I just wanted to make this thread to add a little clarification for how it worked for me for those who search the forum in search of an answer to this issue.

The wire you connect from your head unit to the wiring harness depends on the power from the stock wiring in your car to your wiring harness.

First though you need to find out if the dimmer wire (usually orange/white) going into your head unit is positive. You may have to contact the manufacturer of your head unit to find out.

Red/black on my car from the actual wiring setup in the car (which is a 03 civic lx) is positive so it's a consistent flow of energy. Red is negative and a variable energy flow which controls a variable dim when you adjust the brightness of your cluster gauges. This is usually labeled in your manual's wiring diagram as dash light brightness or something). Whichever wire on your wiring harness that corresponds to that positive wire (orange, or orange/white, in my case orange) is the one you want to wire the dimmer from your head unit to (usually also orange/white) so in my case it was the orange (illumination) wire on the wiring harness to the orange/white (dimmer) wire from the head unit. Wiring the head unit to the wire on the wiring harness that corresponds to the negative power from the actual car is what is usually causing your cluster to fry. Just like with electricity you want negative to negative, positive to positive correct?

In reference to the attached image:

Notice #9 is the positive power source (red/black) that "probably" corresponds to the orange wire on your wiring harness (more than likely, labeled illumination. Depends on the harness, check the labels) double check your wiring diagram (hanes manual is good, your user manual may have a wiring diagram as well) and make sure that is correct with your vehicle. Notice #19 (red) is the negative power source that controls dash light brightness. NO BUENO! If your head unit does not have a negative input that supports variable dimming, then DO NOT wire up to this it will fry your cluster! This is where the majority of horror stories are coming from! I realize this particular diagram doesn't specify + or -. I checked the wiring diagram for my car in the hanes manual, and it listed these values so make sure you find a wiring diagram for your car that specifies whether those wires are + or - it's important!

I did test this once I wired it up to make sure it worked so luckily for me I had an old cluster from when I replaced the cluster (my speedometer stopped working ;p) so I put that back in just in case I wired it wrong and voila! turning on the headlights dimmed my head unit just like I wanted. As a bonus my head unit (Kenwood KDC-655u) had a feature to switch from negative to positive screen color (black background/white letters, to white background/black letters or vice-a-versa) when I turned my headlights on! Awesome!

So hope that further clarifies things!

 
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Not sure what happens, maybe merging wires together. Thinking they need to ground this or that wire or unused wires.... but it sure can make for a disaster when those cluster PCB traces go.

 
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I agree with that. I'm so lucky I still had my old cluster to test with. I already replaced it once and wasn't about to replace the new one.

For those that don't have that luxury and are unsure about their setup it may be best to avoid doing this. Research research research!

 
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good enough to be in DIY section, i would say

 
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Why thank you sir! if you can transfer it to the appropriate section that that would be cool with me. I'm not sure how to do that without starting a new thread and wasn't sure this would qualify as a diy or if it was more just informational. But yeah I hope it helps people out for sure!