If that doesn't work chances are the something has shorted..

Wiring?
Bad Bulb Holder?
Short in the light housing itself?

There could be a leak in the rubber seal around the tail light allowing moisture in. I've seen it before. Hopefully it's as simple as bending the tabs on the bulb holder though.

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Current Bimmer
2004 X5 4.4i Sterling Grey Sport/Premium

Past Bimmers
1991 318I Alpine White
1995 740I Alpine White
1991 525I Schwartz
1998 323IS Scwartz
2004 330CI Cabrio Titanium Silver Metallic
1995 540I Schwartz
2000 Z4 3.0 Titanium Silver
2000 330ci Coupe Titanium Silver