I had a similar problem with the rear license plate light. It turned out that the wires going into the plug for the taillight assembly were mixed up. If the left side is okay, that should give a template as to which wires go where. (I had a similar issue with the left side head light, the wrong lights were coming on at the fog position, I checked the right light, found some differences, made the left one match and voila! corrected it) The grounding wire (brown) might be the place to start. Also make sure all the grounding connections are tight and clean.
If all the bulbs are okay, and the fuses are alright, did you connect the brown grounding wire to the back of the main instruments? The brown wire fits over the knurled nut that holds the keeper (u-shaped bracket) in place. Also, it is important to have the correct keeper on each instrument. The one for the tach has a small bump out that keeps it from touching one of the light connections (my mechanic found this one after much constertnation - and several smoking wires).
Good luck.
Jonny B
1967 250SL Auto