I had a similar problem which I solved on my 300E.... The plastic housing had cracked out around the metal strip which snaps onto the back of the housing. This metal strip must be firmly snapped into the plastic housing. I epoxied mine and taped it while it cured hard. Also there are spring clip electrical terminals inside which may have to be epoxied back if the plastic slots are broken out as mine were. I really wish that plastic had a longer life when used as a structural part!! Anyway, short of buying a new tail light housing, that's about the most likely cure. BTW you did check all the fuses?? Luck to you.

DanielW