Taillights not working

Yes, there is a #9 fuse in this cars that pops. here is a guide to help you test it when you find it from the diagrams below.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the diagrams (below).

Please let us know what you find. We are interested to see what it is.

Cheers, Ken

Okay, so the brake lights work but no running lights. Are the plate lamps working?
GM uses the brown wire to feed the rear tail lights and running lamps. The rear lights meet at a single splice point. The easiest way to see if the issue is the feed wire is to pull out a rear tail light and attach a jumper wire to the brown feed wire and run it to battery positive. If they work the splice (in the rear taillight harness toward the right hand light) is okay and the issue is in the wire before the splice and up to the headlight switch. There is a connector (409) in the middle of the harness at the tail of the truck behind the bumper. It gets corroded inside and the lights stop working. From there it is still a brown wire in the harness that goes up front. You can jump power to the brown wire there as well.
As all the other lights work it is not the switch or fuses as all of the lights are fed from the same feed. If someone added a non-factory trailer harness they may have damaged the harness using scotch lock connectors. That should be apparent if there is extra tape or you see the connectors.