I have fog lights. I gave gone into the bcm with a scan tool and activated the lights and they work. I have tried another switch and still no lights. They stopped working one night on my way home and I did smell something that smelt like hot wires . I have tried relays and fuses all are good.

Given that you have access to an advanced scan tool, and have verified that the lights can be commanded on, I would be looking at surrounding systems that may disable the fog lights in the course of normal operation, such as when the high beams are on, etc.

Is this the 2002 Sierra?

Ok, since you commanded the fog lamp relay to close via the BCM with a scan tool, the only elements left, are the switch(which you replaced), circuit #192(orange wire) which runs between the switch and the BCM, the BCM, and ground G203(but if the fog light indicator came on when you commanded the fog lights on, the ground is ok).....this is the black wire at terminal F of the switch harness....

I guess I would be suspicious of circuit #192, orange wire.......

Go to your BCM, and find connector 3....terminal B5 is the orange wire from the the fog light switch......With ignition on, and low beams on, get a wire and ground it, and back probe B5 at the C3 connector...if the fog lights come on, it'* the wire......rather than find the break, I would just run a new wire from the BCM to the switch, cutting it in convenient places.....

If you ground B5 and the fogs do not come on, there is a problem with the terminal at B5, or the BCM itself.....

I will try that this evening. I did forget to mention that while in the bcm I could see the switch being pushed but not energizing the lights. It read on when let go off.

Here is what I found. With key on lights on dim I have 12 volts on orange wire. . Back prope with ground no lights. Check grounds no issues. . Orange wire has continuity to bcm. Could dimmer switch be a issue here.