If it's illumination, there's only the DRL controller on my wiring diagram, if your truck has DRL. It feeds of the tail lamp fuse, thru headlamp relay No 2, and comes out on red/yellow wire. Check the headlamp Relay No 2 for starters.
If you have DRL installed, short across the Green/yellow to the Red/yellow to take the contoller out of circuit, and see if the lamps bright up.