Quote Originally Posted by Mobius
Not true. Good LED bulbs are quite bright - I wouldn't use them to replace the brake light, but they're more than bright enough for parking and turn.

Unfortunately, the issue is (as I've recently discovered) the sockets on the rear light clusters (with the exception of the brake light) are wired with the incorrect polarity. That is, the body of the bulb base is wired (+) and the contact at the bottom of the bulb is wired (-). LED bulbs require correct polarity as LEDs are, after all, diodes.

You'll have to do some hacking of the wiring on the taillights to fix this. Really odd move on BMW's (Hella's, I suppose) part.

www.superbrightleds.com has everything you need.
REVERSE POLARITY! That's why. I didn't bother checking the polarity