[Favali]

When I purchased my 93 S4 I had a similar problem with the tail lights.

Before I broke out the surgical gear, I took a couple of cans of electrical contact cleaner and starting with the tail lights, I dismantled every light assembly, sprayed, cleaned, and re-fitted the assemblies.

Also, bad bulbs ... as in all 4 trailing lights ... were replaced. Every bulb, ground, & connector that I could find I did this. Including the engine compartment connections.

After that the back-ups lights began to work as did the 3rd brake light. I can say that every light now works correctly. A long afternoon of work so one might need good music, a selection of cold beverages, and patience.