Blown fuse #5
 

1983 911 SC targa, stock, 142000 miles, new to me summer of '13.
With the ignition on Acc or On (engine running): when the low beam lights are on, the right low beam fuse #5 fails when the fog lamp switch is closed. With relay #2 (fog light relay) removed, fuse #5 does not fail.
The fog lights do not light; the left (driver side) low beam lights; both high beams light; the parking lights light; the high beam flash works with the ignition switch on, but not when off. I've searched and looked at the wiring diagrams without an answer. Fuses #5 and 6 (R and L low beams respectively) are bridged behind the fuse block, so how can anything short the #5 fuse without the #6 fuse also failing when the fog lamp switch is activated yet the both low beams are lit fine with the fog lamp switch off? I appreciate help.
Bruce

Do you know whether your wiring is still the original factory wiring? Did the car come originally with fog lights or were they added later?

even though they are both briidged, its the iinput or power source(battery side) that is common. the 2 fuses supply 2 different loads. (i did not read much past your fog light relay. way too much of what works and does not this early in the morning.)
sounds like you are close to fixing it.

Headlights/fog lights
 

If you look at the bottom of fuse 5, you should see a grey wire in addition to the white wire that goes to the headlight low beam. The grey wire is the power source that activates the fog light relay. Normally, they load on the grey wire is very small as all that it does is activate the switch in the fog light relay. Sounds like the fog light relay has gone bad (probably a direct short) and should be replaced. The white wire coming off of fuse 6 only goes to the other headlight low beam. That is why #6 doesn't blow also.

Thank you Fred, PORWOLF, and T77911S.! I will replace the #2 relay and report. Fred, I missed seeing the gray wire invisibly tucked behind the yellow wire to the R low beam and missed how it was fused ahead of the relay on the diagram.
Bruce

Replaced the existing incorrect relay (91161510801 fuel pump) with the correct 91161510901 relay and problem resolved! Thanks, again.
Bruce

Hi, sorry to resurrect a very old thread; I was very excited when I first read this because I was experiencing the same symptom. '82 SC, and the fog lights don't light and blow the #5 fuse. I replaced the relay with a new one from PP, and it's still blowing the fuse.

Is there something else I should be looking for? I don't really need my fog lights, but as I told my wife when she asked why I care, "it's because I have them, and they should work."