Truck shuts off when anything electrical is used

So I have the truck listed above c20 4 speed 305. When I turn on any thing electrical it shuts off the truck. Example of head lights or turn signal. I have been trying to solve this for many months and am just stumped. The battery and alternator are brand new. The truck can sit for months and I can fire it right up but I can't use anything electrical. And yes this includes my brakes because of the brake lights. I am completely stumped.