I have the same issue. Lights are fine when headlights are off. When headlights are on and brake pedal is pushed, driver's side taillight a goes out. I have replaced the bulb and bent the tabs up. Where is the light grounded? Thanks.

 

Hi, I had the same problem with tail/brake light on (04/d2) and was driving me crazy , my brother in law came over and told me that I have a weak ground connection, he checked the ground connection to light assembly , that was good, then he cleaned and scraped the actual connection to light bulb and made it a little tighter by bending the metal contacts and "BOOM" he fixed the problem! He said when there is a weak contact one will work either brake or tail light, but both will not work together, so try that it may fix your problem, if U have a weak ground .