totally confused...electrical

ok my dad has an 04 f-150. It has run great for a long time. The other day he replaced one of the coil packs because it was missing. Now heres the confusing part we cant figure out. He was going down the road and hit the brakes and the radio lights shut off and the odometer light dimmed. This only happens if the headlights are off. If the headlights are on then this doesn't happen. He also has a trailer brake wired up. It has a small yellow light on it. When you turn on the headlights the light on the trailer brake comes on and stays on. It should only come on when you hit the brakes. We have unplugged the trailer brake and that offered little help. We went back under the hood to see if something was disconnected and its not. What is this??? We are thinking maybe headlight switch or bad ground. Remember all this stuff only happens when the headlights are off. Everything is find when they are turned on. So confused. any ideas will help.
thanks in advance

Exactly what I was thinking bad ground. Only correlation to headlights I can think of is it is grounding through it when on. Check all the grounds under the dash. As for hidden grounds I don't know where any are for that truck. Might try looking on Autozones schematics.

Ok we solved the problem. I'm sure this is a one in a lifetime occurance but thought id post the answr in case anyone needed it. It was a bulb. The brake and turn signal are a dual element bulb. When driving my dad noticed when he hit the brakes one light seemed brighter than the other. He got to work and looked at it. The turn signal element of the bulb broke and landed on the brake light element of the bulb. They were fused together.It was feeding electricity back into the system is the best to think. He changed the hulb and the problem is solved

Odd problem, impossible to diagnose that without just throwing parts at it. Wow! Glad its fixed though..