I would suggest:
Remove the radio.
Access the electrical plug for the radio.
Insure that, with truck OFF, key OUT, you have power and ground where needed.
Start the truck (or, at least put the key in RUN), and make sure you still have power and ground where needed.
Than, turn on the headlights and see what changed.
Continue from there.
BK/YL wire tells the radio that the lights need to come on. You should have voltage when the headlight switch is in (parking lights) or (headlights) position.
OR wire tell the radio to dim/brighten the lights. Voltage should vary in (parking lights) or (headlights) position, to correspond to your dashlight brightness.
RD/WT wire should have voltage ONLY with ignition switch in ACC/RUN position.
PK wire should have voltage at ALL times.
Ground is provided via a ground strap, or separate screw-on wire.