1985 Plymouth Gran Fury brake lights

[RARoMiNous]

So the other night I got pulled over for the first time on account of my brake lights not working. I was pretty perturbed seeing as I had just replaced them less than a month ago and so I took a look at them. What I discovered is that my brake lights work when my headlights are off, but not when they're on. I have no idea what the problem is, so any help at all would be appreciated.

[jdl]

In the diagram, Check the ground circuit wiring. If problem with ground, the tail lamp maybe trying to ground through the brake/turn voltage circuit. If you can't read the diagram, Gray wire is ground. Black wire is voltage tail lamps. Brown wire is voltage for stop and turn.