Headlights stay on

Sounds like something is keeping the Headlamp relay closed. Could be a bad relay stuck closed. The two inputs are from the Body control Module, but that should loose power when the ignition switch is off. The other is the Headlamp and Panel Dimmer switch. This is always hot. Remove your relay from the Underhood Bussed Electricl centor, and test it out. This diagram is hard to read, but the pin numbers should be 30, 35, 36, and 37? Anyway, pin 30 and 37 should be open with the relay out. If they show continuity, then the relay is stuck closed and needs to be replaced.
Now, if that leg of the relay is open, then check pin 35 for power. If it has power, I would suspect the light switch in the truck as being bad keeping the relay powered.
Good luck and let us know what fixed it.

I would check pin 85 for power. If you have a volt meter, touch the pin 85 with the positive lead, and the negative lead to a known ground like the frame. If you are showing power, like 12 volts, then I would suspect the light switch in the cab to be the problem.
If you dont have a meter, most auto parts stores sell a light tester that will light up if 12 volts is applied. Touc the positive of the light meter to the pin, and the negative to a ground.
I know the older fords usta have problems with their light switch turning the lights off while you were driving, but I guess GM does the opposite. Lol