You might have better luck tracing the problem with a multimeter. You can hopefully locate the problem area and repair or replace the bad part.

As I mentioned before, the problem I had was a small hole in the metal plate that the lamp socket (should have) made contact with. I didn't notice the problem until I dissembled the tail light assembley.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles