Taillights do not work, Brake warning light flashing?

I would start by checking the fuses (park lp 20 amp) here is a guide to help and the fuse locations in the images below. Also, I have included the taillight wiring diagrams so you can see how the system works.

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. Here is the fuse panel as well.

Okay, I checked the fuses, all have continuity. I think the brake warning, "Code 9 " and the taillights are two separate issues. I was thinking the taillights were a grounding issue. I just had the truck repainted, and I checked grounds, okay. I wonder if I may have damaged the headlight switch when I pulled the instrument cluster to fix gages.
Only lights that work on the rear are turn signals... but only if the headlights/parking lights are not on. If I turn lights on, none of the turn signals work. Note: Headlights and front parking lights do work.

The front and rear running lights work off of the same circuit, I would check for power on the brown wire in the loom headed for the rear lights.

Okay, Problem solved. The "Code 9" was not related to the taillights. It was caused by swapping the instrument cluster "Hot". I verified it was seated correctly, and pulled the neg. batt lead and the code reset. The "gremlin" in the taillights was due to bad ground. When the body shop pulled the bumpers to repaint and replace, they broke the ground wire to the frame and didn't fix it. Once I discovered the break, and spliced the wire and grounded the lights, the issue has been resolved.

Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.