No lights on rear of truck when headlights are turned on

I have a 1993 Chevrolet K-1500, 4x4, Automatic Transmission with a 5.7L, 350cid v-8 throttle body fuel injection. The truck has approx. 260,000 miles on it. When I turn the headlights on, the whole rear of the truck stops working, no turn signals, no brake lights, and no license plate lights. But when the head lighs are off everything works fine, but if you engage the turn signals no matter which way you select and then hit the brake pedal, the flasher starts to blink at a medium speed, (not as fast as with a blown bulb, but faster than normal) and as soon as you let off the brake it slows down to normal speed. Is there something I should be looking for other than a ground malfunction, if so or if not where should I start looking? Please help hair is getting thin......... Head lights always work and the bulbs in haven't been changed in 3 years and this just started happening a week or so ago.