Sounds like you're going to have to start by getting a tester (if you don't have one already) and check the light sockets for power. AFTER checking fuses.

A simple tester like this is all you need;
6 12V Circuit Tester with 12 Ft Lead

IF there's no power to the sockets then there should be a connector for the tail/brake lights under the rear of the truck. Check there for power for the lights. If no power then trace the wires along the frame to see if you may find pinched or broken wire(s).

Electrical problems can sometimes become a challenge. I try to go about it in a methodical manner and not 'helter skelter'.

You've already changed the bulbs so those shouldn't be the problem. Now you need to check where the power stops in the circuit.

i hope this helps.