I understand that this is definitely a wiring issue, however where I am stumped is it was working just fine until I replaced the tail light lens. Where would one start the diagnosis at? I turned the lights on and put my tester on the brown wire close to the bulb and of course nothing. Any "easy" fix is appreciated!

 

Need more info...
Is it just 1 taillight not working? Both taillights? Front parking lights?
All the other lights working?

 

I actually went back to work on this truck after posting and found the culprit. Wires had come apart under the back end. Got them wired back up and now all is working! thanks for the response!