wiring trouble

[65truckowner]

I am trying to finish up my truck and am having wiring trouble. I am using the used harness. It looked good so I checked and rewrapped it. It will start and run. but I cant get the tail lights to work. I have power to the back of the truck until I touch the wires to the taillight wires then my test light goes dim. I started back tracking and it is dim all the way to where it plugs into the firewall. With the frame harness unplugged the light is still dim until I start moving wires then the light goes back bright. Seems to me like a ground but what is grounded under the dash. Also I have no brake lights. It goes through the brake switch but does not make it to the plug at the fire wall, Cant get my park lights to work again my test light will light up but can get the new bulb to work no signals at all. I am thinking about buying a new under dash harness what do you guys think