Those heaters draw 70 watts. They have 10 amp fuses. The radio pulls more current.

The heaters are not connected to the factory harness. The truck was fine on the drive in and fine when I went to lunch. It was not fine after work.
Since I was more concerned with the poor road conditions than the running state of the truck I may have overlooked a charging issue earlier.

I'm thinking it's a ground or alternator problem. There's no reason for the heaters to make the brake warning light come on when I hit the window switch.
I did roll down the window that morning so the problem started after lunch.