92 GMC, my taillights stay on whenever I turn on the truck.

On my 92 GMC, my taillights stay on whenever I turn on the truck. When I press the brakes, the brake lights do not turn on. Every time I turn on the headlights, the taillights turn off, nothing works. I have checked all fuses, they are good. I also changed out the brake switch. I can't seem to find the issue to this problem. I have also noticed that the middle bulb on my taillights are getting hot to where it melted part of the plastic cover. Im at a lost, does anyone know or have any ideas how to fix this issue?