I'm installing a set of driving lights on my 2000 and I'd like to use a factory switch to control them. The truck did not have fogs or the wiring from the factory.

The switch that came with the aftermarket set has three wires going to it. The only connections that I made to get the lights working were red from the relay to fuse block power and black from the relay to ground. The lights work with the switch that came with the set.

I bought a headlight switch at the wrecker that has the fog switch with it and the pigtail has four wires...black, orange, purple and grey. Anyone know if this will work and how I should wire it?

 

I've attached a page from my 1999 service manual that I scanned for myself a while ago.

This is my understanding of the circuit:
The fog-lamp switch itself does not latch open or closed, instead it only temporarily closes and reopens when released. It is wired so that when depressed, it temporarily grounds the "Fog LP Request Signal" input on the BCM which then commands the "Relay Control" circuit in the BCM to energize the the Fog Lamp Relay (I think among others). This closes the switch on the Fog Lamp Relay supplying 12V to the Fog LP Indicator lamp and to the fog lamps themselves.

You should be able to confirm the operation of the switch with a multimeter and the schematic I attached.

From my understanding, because the OEM fog-lamp switch does not stay in either the closed or open state and always returns to the open state, the switch itself will need to be replaced or modified. Also, being a logic switch, it is likely not designed to carry much current, not even enough to energize a relay.

One solution might be to use a latching relay to control the fog-lights.

However, if you can figure out how to make your installation look stock, I bet many people would love to see how you did it.

 

Spent some time digging around learning about latching relays and I set up a system that I found on the12volt.com but it still isn't working. Here is the diagram https://www.the12volt.com/relays/rel...oogle_vignette

And here is the modified diagram with how I interpret that the switch should be wired. Any ideas what I'm doing incorrect?

 

OK....now I have looked-up the relays and found what terminals are which.
Coil is between 85 and 86.
Switch pole is terminal 30, NC is terminal 87a, NC is terminal 87.

So now my question is does the factory fog lamp switch purple wire go to ground/12V return when pressed? This is what provides the coil in the left relay the path to ground/return.
The right relay as shown has the coil powered all the time, so it is always in the switched position...or ON. The left relay is doing nothing.

 

Funny, I think I might have come across that same diagram.

For an application to override my daytime running lights (DRL) that I've created, benched-tested, but have not yet installed in my Blazer, I used a Flip-Flop latching relay I found on eBay to control the relays that would cut out the signals to the Body Control Module (BCM) that control the DRL system. (see attached)

The only part my wiring diagram that would be of use to you is in the upper left-hand corner where the Flip-Flop latching relay is controlled by a push-button that temporarily sends a ground trigger to the latching relay control.

Here is a picture (below) of one from eBay that looks like the one I used. (https://www.ebay.com/itm/185441414855)

The only other thing from my diagram you might want to consider is to tap into the Retained Accessory Power (RAP) 12V supply so that you can't leave your fog-lights on by accident. I tapped into the RAP supply to modify the power to one of my accessory outlets so that it is only on when the RAP is on.

I hope this helps.

Once you figure it out, I'm sure others would love to see your solution along with me!

 

So far, no go with figuring this out. Electrical is not my strong suit tho. Found this video which seems to be a valid option. Just need to find the proper BCM and run the fog light harness. Doesn't seem too hard. Wrecker BCM's here go for $50. Anybody ever tried this?

 

Just for anyone looking in the future, that video lays out the steps pretty good. I found a BCM at the wrecker with the same part number but the vehicle came with foglights so grabbed that plus the front headlight harness. Install was easy, took a couple hours including the relearn procedure. My truck already had the wiring in the dash and the fuses/relays in the underhood fuse box. Just changed the switch, changed the BCM and front harness and did the passlock relearn. Only issue was that the airbag light stayed on once I got it running so I went back to the wrecker and grabbed the air bag module from the same vehicle from under the center console and now everything is working properly.

 

This is really good information to know. I've always wondered about how specific each of the BCMs might be.

That is interesting about needing to also get the airbag module.