1985 Plymouth Gran Fury brake lights

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.

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.