I have a 2005 4.0 4x4. With the head lights off everything works normal. However, when I turn the headlights on the brake lights stay bright as if im on the brake, including the third brake light being on. I thought about a bad switch BUT the left turn signal does not work when the headlights are on either. The arrow on the dash just lights up and stays on. Any ideas? The only thing I have done to the truck in the past year is the fog light mod a few months ago. Making it so the fogs stay on with the high beams. I'm going to try to reverse it back to stock to see if that helps. Do any of you guys have any ideas?

 

The left brake light bulb is probably shorted inside or the socket.
Also check the wires and plug for the factory trailer light harness.

 

I actually got it fixed right after I posted that. I went out and started fooling around and found the trailer harness to be shorted. All the salt and ice just corroded the crap out of it.