2012 389 lighting schematics

I may be wrong on this, so verify it. I have not gotten into Peterbilt multiplexing too deep, but this is how Freightliners work.

A known good switch may not be the right switch. You may need to buy a fog light switch. The computer looks for a signal from the switch to activate that circuit. Mutiplexed switches have an IP address so to speak. If you unplug your headlight switch and plug it into a different plug, it still works the headlights. The switches hook to a data link, think of it as a USB key board. All those switches (keys) work through 4 wires.

Another thing that can happen is that the computer will disable that circuit if amperage is too high or too low. I've had instanced where a light was out and replaced the bad bulb and it still did not work. The problem was that I had the lights on while putting in the bulb and the ECU had already shut it down for an open circuit. All I had to do was shut the lights off and turn them back on and the ECU would try again. Once it sensed the correct load, it worked fine.

 

View attachment P94-6147.pdf

Here's a schematic of your exterior lighting. It look like the fog light swtich has a separate input on the Namux connector 4, pin 8. So verify that the pin 8 has power with the switch. That shoud then power up Pin 15 on chassis node connector B.

This is a little tough to diagnose without ESA, the Namux is very hard to get to. With ESA, you can see if it's getting a signal or not. If it is getting power from the fog light switch, but not activting the fog light circuit, then it may not be programmed.

Dumb question, if it didn't have fog lights from the factory, does it have a fuse installed? Look at the schematic, that plug has is own fuse, # F4.