Do you find that every now and then a main/fog front/rear bulbs suddenly turns off, even though you know the bulbs to be perfectly OK, and perhaps even brand new? Then this fix is for you! The problem lies inside the fuse box in the lower right corner on the above picture.

Remove the plastic lid to gain access to the fuses and relays. On this photo, the relay for the parking lights (including tail lights) are the yellowish one in the middle, right above the big black modules, part no. 1 378 052. The larger of the modules, the one at the bottom, are the one that controls the headlights and the fog lights.
Open cartridge to reveal the relays. Fire up your soldering iron and resolder all joints. Clean all contact points with electronic cleaner spray. If you are able to get a piece of paper in between the contact points and move it around to remove the now loosened dirt and leftover cleaning agent, even better.
The orange relays are not serviceable, but all the others have been opened and cleaned on my car. I think it is worth the time to clean all of these, that controls things like the auxiliary coolant pump and the blower motor.