2001 Rodeo brake light problem

My 2001 Rodeo's brake lights work with the headlights off, but go off when the headlights are turned on. (The 3rd brake light works either way.)
Also, the cruise control won't engage when the lights are on, but works fine with the lghts off. I had a trailer light converter installed, but the trouble remains when I disconnected it. Any thoughts? Thanks,
MikeB666

I saw this problem before on either this board or planetisuzoo.com (do searches on both boards to see if you can find it). I dont remember what it turned out to be, if anything was found at all. A problem with the brake light circuit will cause the cruise control to not work since they are connected together.

It may not be kosher to respond to my own question, but here goes. First, thanks to Gizmo42 for good references. I removed both tail/brake light bulbs and checked both elements for continuity. All was good. I reinstalled said bulbs and brake lights and cruise control work as the Good Lord and Isuzu intended. There must have been a bad connection in one of the light sockets,probably to ground.

Thanks,
Mike666

Glad you found it and it was a simple fix. Have to try and remember that the next time the question comes up.