DIY: Rear Fog Light Wire Via BCM and Stock Wiring

spearfish25

An ON-OFF switch can be used, but it will be annoying if you wire it the way I describe (controlled AT the BCM instead of AFTER the BCM). My DIY describes how to tell the BCM to turn the rear fog on/off so that you maintain the factory interaction between the headlights and rear fog. IE, the rear fog can only go ON when the headlights are on, and goes off if you turn the headlights OFF. The way the BCM reads the On/Off signal is momentary. Imagine you have two bare wires that make up the On/Off circuit instead of a switch. The BCM just looks for the two wire ends to briefly touch, and it turns the light ON. Another brief touch then turns it back off. This is what a momentary switch does. An On/OFF switch is like keeping the two wires touching when it's ON and having the two wires separated when it's OFF. However, once the BCM gets its momentary 'touch' input, it doesn't care if the wires are still touching or not. What this means for an On-Off switch is as follows. Assume the headlights are ON. If the fog is off and you switch your ON-OFF switch to ON, the rear fog will go ON. If you then switch the On-Off switch back to OFF, the rear fog will stay ON (the switch doesn't give another 'touch' when going to off...it's just like you touched the two wire ends together and delayed while separating them again). You'd then have to switch the On-OFF switch to ON again to get the rear fog turned OFF. Thus, using an On-Off switch instead of a momentary would mean you have to give it two switch throws every time you want the rear fog to go either On or Off.

Short answer....find a momentary switch and follow my DIY. Option 2 is to wire an On-Off switch from a 12V power source to the orange fog light power wire. You'll lose the headlight dependence, for better or worse.

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2013 Cadillac V-Wagon, RIP Z