Brake lights on 89 plymouth sundance?

The brake lights only are dim on the 89 plym.-its not the bulb. The rear window brake light works fine-there is a trailer hitch on the car, but it doesnt look like there were any kind of lights hooked on to it. Now we have to get it fixed to be in compliance but where to start? When the headlights are off and you step on the brake just the tail lights come on, not the brake lights just the tail lights-but when the headlights are on and you step on the brakes the tail lights go out-with no brake lights on either side.