Stereo & Electronics - 1 brake light dont work when lights are on.

Both brake lights work fine until I turn the lights on, then the passenger brake light doesnt work. both tail lights come on , but the passenger side doesnt light up for braking. Any ideas?

Sounds like a bad ground wire on that side. When the normal ground path is broken, the bulbs will backfeed to ground through the other filament. When the parking lights are on and the brakes are pressed, there is 12V on both sides causing the lights to go off.

I had the same problem once, and it was because I didnt use a dual-filament bulb on one side.

the ground wire is the same on both sides(splits from 1 main ground to 3). there diffinetly dual filament bulbs.:confused:

Did you test the ground or are you just assuming it's okay? The fact that it joins another ground wire at some point does not mean it's a good ground. The wire between the sockets and the splice could be broken or the sockets themselves could be corroded or damaged. Always check the basics before trying to find some more exotic cause of a problem.