Why does my brake lights turn on by themself.

check the clearance of the brake lite switch. If it is resting on the brake peddle then just the vibration of the truck will cause the lites to go on and off. Increase the distance between the switch and peddle arm and it should solve the problem.

Thanks for your response Tristar0101. I tried but no change in operation. One thing I did notice is that the malfunction only happens when the ignition key is turned forward. It does not matter if the engine is on. I am wondering if it is a computer or some kind of module problem or something else.

Sounds like a short in the system somewhere or a loose power wire.

Yes, I think so. I am going to break down and take it to the shop. I'll repost what they find. I appreciate your help. Thanks!

OK, I found out the problem w/o going to the dealer. On the brake switch wire harness there is a white wire with a red stripe. The wire is loose inside the harness. When I move this wire back and forth the rear brake lights, not the 3rd brake light, comes on and off. I plan to take the harness apart today and repair the wire inside. Thanks tristar0101 for your help.

I thought so...it sounded like a loose connection and I am very glad you found it. Wrap it up tight so you don't have a repeat...that would be a bummer for sure.. Very glad my tiny input helped you find the problem.