That is why I did that. I used a "Tap a Fuse" to tap into one of the high beam cicuits in the underhand fusebox to power the coil on the momentary relay with the power for the lightbar circuit routed through the contacts and then on to the lightbar. It is pretty simple.
Just did this yesterday on a '18 ZR2. We went through the hood latch rubber gromet. We installed a factory headlight switch with the fog button and ran a 22 gauge wire through the gromet to a latching relay since the fog button provides momentary ground. This means your latching relay needs to be able to trigger on ground. We did this install for a lightbar and interlocked it with the highbeams, so the lightbar could only run when the brights are on. Fogs are legal on highways, but lightbars are not in most states. Use them at your own risk.
It is actually a simple install if you take your time and think it through. Watch Tinkering Fox's video 2 or 3 times. If you are handy with a soldering iron, make your own harness. If you want Plug-n-Play, then order a harness from Phil Gamboa. Main thing to be careful of, the driver side interior fuse panel cover can be difficult to remove, so don't get too aggressive. Make sure the terminals you put into the headlight plug, BCM X5 and X1 plugs are oriented right, otherwise you may not get good connections and the frustration sets in. Keep in mind that the signal from the headlight switch to X1:12 supplies ground to the BCM. With that input, the BCM then supplIes +12v to the X5:5 terminal to light up the back light indicator on the headlight switch and the BCM will also supply a software command to light the off road light icon in the instrument panel. The wire that goes to the headlight switch back light indicator (yellow wire) is also run into the engine compartment and supplies +12 volts to the off road light relay. The BCM latches or maintains the signal, until the headlight switch button is pressed again. Keep in mind that the off road light button on the headlight switch must be pressed for 4-5 seconds to make the BCM activate or deactivate the signal. You can also interlock this signal with an interposing relay triggered by the high beam relay so your off road light only runs when your high beams are on. Pay special attention to Tinkering Fox's description of unlocking the tpa's on the all the wire plugs in the YouTube video, otherwise you may not get good connections at the various wire terminal plugs. Good luck!
My hat is off to Tinkering Fox and Phil Gamboa. The video is fantastic. I have a 2018 Sierra SLT and ordered the switch and made the harness (sorry Phil). At first it didn't work, nothing coming out of the X5:5 BCM connector and no indicator light on the instrument panel. After checking continuity I found that I had to reorient my new X1:12 connector. I had to turn it 180 degrees in the X1 (white) plug. Evidently it wasn't making good contact with the BCM pin. Once I did this, everything works like it should. If you have issues, check for continuity at each connector. You can check continuity on the ground (-) signals with you multimeter. The yellow wire in Tinkering Fox's drawing X5:5 is a +12v signal. Keep in mind that the BCM triggers (turns on) with a ground, not a positive voltage. I also installed an interposing momentary relay for the high beams to interupt the power to the off road light, so there is no delay in killing my lightbar when meeting oncoming traffic. I would still like to know how Tinkering Fox knows what functions are in the factory BCM programming and the pin placements. I can't find any of this information in the Upfitters guides. Great job guys!
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