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!

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