Rear lighting problem!

[70-Chevy]

ALright here it goes.....

When the light switch is OFF, all my park lamps work, headlights, brake lights, everything like it should.

When the switch is engaged to PARK LIGHTS and HEADLIGHTS, the whole back end of the truck goes out. The brake lights wont work, neither the turn signals or park lights.

The whole front end works all the time no matter what position the switch is in.

I just put in a new switch and troubleshot the wiring harness all the way to the rear harness plugin. Everything works like it should according to my test light, with the rear harness unplugged.

I thought it might be a bad harness, but everything works fine when the light switch is OFF, so the lights MUST be getting power correctly.

ANY suggestions would be awesome, thank you all!

I

[ChevLoRay]

It's grounding that is the problem. Whenever you use a test light, you ground it. When you do, the circuit is completed, as far as the test is concerned. But, after almost 40 years (at the most), rust creeps in and spoils the grounding that was part of the truck when new. So, look at all of the grounds and re-do them. Take the connection loose at the chassis, and at the body, clean them up with a small brass bristle brush, including both the chassis and the connector. Be sure the lock washers are clean, as well as the nuts that hold them intact and tight.

When you have redone all of the grounding points, see if it isn't better. The voltage will be present in the wiring, but the lack of a good ground will ruin it all for you.

[truckdude239]

i had the problem with my lights in the backs not working take out the buckets for the back lights clean the bolts and the sides of the buckets where they touch the body. if that don't work, unplug your turn singal switch and plug a new one in and see if that fixes your problem