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

[ray_mcavoy]

The dim test light is an indication of extra resistance in the circuit that could be due to a corroded or loose connection. And the fact that it brightens up when you start moving wires around seems to point toward a loose connection. Are the wires you're moving the ones near where the harnesses plug together at the firewall? If so, I would suspect a loose connection at that point (possibly a bent / distorted terminal inside the connector that isn't making good contact).

The wiring for the brake lights runs from the brake light switch to the turn signal switch. Then from there to the firewall connector and on to the back of the truck. By passing the brake light wiring through the turn signal switch, it allows the turn signal to over-ride the brake light on the selected side. But it also means that a problem with the turn signal switch (or poor connection where it plugs in) can prevent power from passing through.

I'd recommend giving the existing under-dash harness a good thorough inspection looking for damaged / corroded terminals in all of the connectors. If you find a terminal or two that's bad, they can usually be replaced without much trouble. But if you find lots of corrosion or other damage, it would be a good idea to replace the harness. Although that alone might not cure all the problems if you're also dealing with bad switches and/or issues with the other wiring harnesses.

[65truckowner]

When I was moving the wires the light brighten up once while moving wires above the column the next time it was about the fuse block. I did inspect the harness but like you said could be the connections. Also I unplugged the column harness and had nothing while the brake pedal was down. The power did go through the brake light switch. 300 bucks I can get a new under the dash harness and turn signal switch