check the wiring harness and contacts. i was getting the same issues with my brake lights ... bulbs were good, contacts good, it ended up being the wiring harness not being snug enough in the clip.

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2004 X5 4.4i, Sport Pkg
2004 Tacoma Xtra Cab 4x4, 9" lift
2001 Passat, 1.8T
1995 Tacoma Xtra Cab 4x4, 3" lift
2002 GSX-R600
1987 Rebel 450C
2000 E-Z-Go TXT