I'm not going to read through 5 pages of posts, I'm just going to say how I fixed it on my X5 and 2 of my e36's in the past.

If you take out the tail light and look at the plate that the bulb that is not working connects to, you will see corrosion at the point where the bulb holder makes connection with the metal plate.

All you have to do is drill/drimmel out all of the corrosion. I drimmelled both of my e36's and drilled my e53's. Once it is clean, use some thicker paper/thinner cardboard and put it between the metal plate and the tail light.

Then use solder and fill the hole. The paper/cardboard will keep it from dripping through to the plastic or down into the tail light. Once dry, drimmel it smooth.

Everything should work as long as the bulb holder is not corroded. I have drimmelled the contacts on them before just to help, but if they are too bad they aren't that much to replace.

I used to have pictures of the whole process, if I can find them I'll post it.

Hope this helps someone out there.