My 2002 Silverado has it's rear drivers side tail light out. I replaced the bulb and that wasn't it. In fact the same bulb works when brakes are applied? The rear passenger running light works fine as well? I checked fuses and they are all good? I'm at a loss? The light went out originally, and when I replaced it with a new bulb the light was very faint, and then slowly dimmed out?? Now it doesn't work at all?

I'm guessing you have a faulty lamp socket. You may also have inadvertently installed the wrong bulb or perhaps installed it incorrectly.
Those trucks have been known to have less than robust lamp sockets so I'd suspect that's going to be your case.

Thanks. I'll give it a try.