Just to make it simple...

1990 S5 vert.

Fog lights work fine until I drive in the rain.
I have done the H3 bulb conversion as the standard replacements bulbs are nowhere to be found.
There is no water intrusion into the fog light housing.
Tested the circuit between headlight switch and fog light relay, it checks out.
Tested the circuit after the relay, tests fine.
I can replace the fuse and fog lights will work perfectly fine until it rains.
When the fuse blows the fog light relay still works, so we know it has to be the circuit after the relay.

Even physical inspection of the fog light wiring after the relay looks good, so I am stumped!

Thoughts or ideas I haven't already covered?

 

Moisture is most likely getting in to the circuit somewhere some how to ground out and pop a fuse . Don't remember if the cpu has to do with lighting or not . Possible faulty switch ? So you run with fogs on other then when it rains ?

 

its a good thing they work when its foggy or you'd be in trouble!

so i looked at the die-a-gram and the fuse is on the R/Y wire which is always powered from the main fuse, and just goes right to the fog light relay, and then the bulbs, and then ground.

i would guess you're looking for continuity of that wire to ground, when its wet. i think the wire goes from the main fuse block, to the fuse block under the dash, and then back out to the front, so i think i'd pour water on the car, and see what gets wet

 

Where does it ground to the chassis? I think you are on to something.

 

Edit: Deleted for being totally wrong.

 

the ground for the power side of the fog lights, ie the wire that actually lights the bulb looks to be the one under the airbox.

i think to blow the fuse the circuit needs to draw more current, so the short must be before the bulbs, but that is just a guess.

 

Yeah, looks like JC-04, a five (black) wire gang connector, behind the headlight and below the airbox.

 

yeah 4, i think i just had this off of mine, and i recall just the one wire, but i could be wrong. BTW i almost ended up running that whole harness though the old opening from the air pump dump hose, it only does the lights so it doesn't really needs to go up and over it can go under. i didn't do it, because i think my IC is in the way, and i didn't suggest it to you Clokker, because you've got that engine cover

 

Before you go tracing every inch of the wiring, narrow down the area.

Turn the ignition on, then turn headlights/foglights on. Grab a hose. Start at the fog light and spray water around each one as well as underneath. After that, spray against the windshield so lots gets down into the cowl. Then if it's still not blown, spray down the body harness.

The idea is you can narrow down where water is leaking in by spraying water in, and when it shorts it will blow the fuse and the lights will turn off. Easy peasy.

 

agreed, we know which wire it is, and now to find which part of that wire. i suspect it leaks by the interior fuse box, but somewhere in the foglight harnii is a good candidate too

 

I'm guessin' one of the bulbs is getting water on it and blowing the circuit.

 

I might start with pouring water on the windshield then . I suspect it may be leaking in either past the windshield seals or the cowl. Have you pulled back the carpet to look for wet floors ?