My left rear tail light doesn't come on when my head lights are also on and the transmission is in drive. The light works when the headlights are off and the transmission is in drive.

I've removed the light there doesn't appear to be anything wrong. Same story with the relevant fuses.

Have any of you encountered this before?

Thanks for any help you can provide.

 

Yep, happened to me a few years ago. Long story short - check the little ground pins and springs in the sockets. It's not on with both running and brake lights on because your brake light isn't grounded, so it's grounding through running light.