I have a 93 500SL. On my instrument cluster, I have the light on for an external light out. The right side tail/brake light has only 1/2 of the light lit up. It takes a combination bulb. The socket has only one lead, while the left side has two. Does anyone know why this is? It doesn't make sense. Here's a pic of what I mean.

 

I would make sure that you have the correct bulbs installed first. Also make sure you have the correct bulbs in the correct sockets. left side inner most bulb is for the rear fog light, I think anyway. the outters are the turnsignals. and the center bulbs are brake lights. I do also seem to remember the tail lights had ground issues with the wiring or the bulb carrier it self was bad.
you also should have rear side marker bulbs too make sure they work.