Oh bummer man.
Well, the taillights not making great contact with the housing (from the start) surely doesn't help...
http://www.xoutpost.com/bmw-sav-foru...ing-fixed.html
Checking the sockets back there for corrosion couldn't hurt. If all else fails, just carry this....
RealOEM.com * BMW E53 X5 4.6is FIRE-EXTINGUISHER POWDER
j/k