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29mdla
03-12-2008, 15:01
Is there anywhere in the cab where I can find a wire that that will provide 12v for gauges that will be on/off with the headlights? Under the hood? This is for a 2008 2500HD GMC.
DmaxMaverick
03-12-2008, 15:18
Depending on where you need it to run to, there are a few options. For aftermarket gages, you can get dimmer (with the instrument panel) voltage from behind the headlamp switch. There are other places in the harness, but the switch circuit is easiest to identify, and access.

If you need it underhood, and don't want it dimmed, tap the marker lamp circuit (non signal).
29mdla
03-12-2008, 16:37
Know the color of the wire?
DmaxMaverick
03-12-2008, 17:07
No. I don't have a 2008 truck or diagram. You can find it by probing the rear of the switch with a test lamp or meter. It will be the one tracking the dimmer knob.
29mdla
03-13-2008, 09:30
On the 7.50-08 this will not work. The switch sends signals to the BCM which then turns on the lights through relays that are somewhere. I found I can use the front Park Light fuse in the under hood fuse box for power. This turns on/off with the head lights, but no dim.
DmaxMaverick
03-13-2008, 10:33
The circuit wire you want is after the BCM/relay. Same circuit as the park lamps, but go to a different relay for the IP lamps. It powers the IP lamps directly from the rheostat (actually a resistor PCB) in the headlamp switch. The lead at the fuse panel will not be dimmed. I've done this on later models. They are different than earlier models (different wire color and location), but the output is the same. I've installed up to 4 additional gage lamps on this circuit with acceptable resultant illumination. Too many lamps and they all fail to illuminate high enough.

I had looked over the diagram for a 2007 some months ago, but don't have one on hand. It is possible it changed for 2008, but I don't think it did. Did the IP lamps get changed to LED's? I've heard this, but haven't been able to confirm, and the vehicles I've seen did not have LED's. I'll get my eyes on the diagram and verify all this.