Have all lights in cab and on rear of cab. No rear brake light's dim lights in rear and no running lights had them and all of sudden gone. When blinker on work in rear until i turn headlights on any help would be helpful thanks ALL FUSES TEST GOOD. GOING TO REPLACE HEADLIGHT SWITCH HOPING

 

Before you repl the headlight sw, consider checking the ground connection for the rear which should be located inside the cab ,driver's side rear, of the cab. If that is too difficult to locate, try running a long piece of wire from the battery negative terminal to any one of the rear lights ,tail/stop and see if there is any difference. It does sound like there is a poor ground and the minute you place a load on the circuit the higher load is robbing the ground.
There are two things you should consider and one is to confirm you have full voltage coming to the lights when they are supposed to work. If not , you need to back track to the source and see where you are losing the power. What we do sometimes is to supply a fused line to the lights and confirm they work and have a good ground. If they do , now you know there is a power drop somewhere and you just need to back track to the source and with that process you will find the problem.
Do you know if the windshield ever leaked on your vehicle, especially in the lower driver's side where it can and does create some wild problems with the electrical components there.