Rear lights on one side not working.

BCM

You could have bought a multi meter and learned how to use it before you went to all the time and expense to replace a wiring harness , Now you're back to square one and still need a multi meter

Trucks throw a lot of gunk up into the back of the rear fenders and bumper where it sits and holds moisture, and twenty years of this will do it. This often causes rust, especially around electrical devices. I suspect that the ground to the left rear has rotted out and needs to be fixed.

Yep that’s why I bought inner fenders for my truck. They were the pits to put in but now my tail light assemblies stay dry from the back side.

Site error

Check your tail light module, it is attached to the rear frame and your tail light harnesses plug into it.