When the trucks running no lights either? Does the truck take a long time to start? I would agree to check the relay. Switch it with another one and see if it fixes it. It sounds as if u are getting a voltage drop or you are losing the ground when the accessory is energized. Try disconnecting one of the headlight plugs and see if one light will stay on. Can you pull the headlight switch and check it with a meter with key off and on to see if the problem is showing up there.
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