Cadillac: 99 Cadillac ETC, signal work, tail light

Question
My left tail light works ... sometimes ... sometimes not.  Cannot trace the problem.  Bulb OK.  Fuses OK.  Can you help by suggesting somethings to check?

Answer
try another bulb as there may be an intermittent faulty connection inside the bulb housing where you cannot see it.

Does the turn signal work OK ?.

with the lights ON, wiggle the bulb socket in the tail light as it may be an intermittent faulty socket that needs to be replaced.