In my case this was caused by a short to ground in the wiring that goes to the right rear lights. An important clue was that only the right rear lamp did not function (in addition to the cluster lights, I mean). The left rear worked fine. The wiring diagram was a big help here. Fortunately, my car was already half torn apart for renovation anyway. I hope you do not have to do the same.

Rich Mayer
82 500 SL Euro