sounds like its a good thing you picked up a new switch....

 

Tried the new switch, exact same problem.

Pulled both tail lights, everything seemed okay there. Also looked at the wiring under the box of the truck and its all together there too.

Gonna have to pull out a wiring diagram and see whats going on I guess...

 

WELL GUESS WHAT? This was my problem too!

After various tests we traced the shorted section of wire in the harness between the engine bay side of the firewall and the driver's fender. Somewhere where the split loom tees into the harness that goes through the firewall. Had it traced down to about a 12inch section of the harness. Ended up cutting at both ends of the 12inch section, and running new pieces of wire down to the connector behind the drivers fender/ body mount, and up through the firewall grommet to the bulkhead connector under the dash. It was much easier to do it this way due to the old gooey hockey tape-looking-tape used on the harness where it goes through the grommet, probably wouldn't have come out very easily.

Wow, is wiring ever fun. Now if I can figure out why my cargo lights and interior dome don't work, I'll be set!