My 93 s10 pickup’s turn signals blink really slow. Also, when I step on the brake pedal, the turn signal stops blinking. When I turn the headlights on, the tail lights and brake lights do not work. Finally, when I do step on the brake pedal when the truck is off, the dash lights turn on also.
This sounds like a bad chassis ground connection. For some reason, the brake light circuit is seeking a path to ground through the dash lights. I would start by checking for a good ground at the rear of the truck for the tail lights and brake lights.
I had another thouht: if either one filament in a tail light is making contact with the brake light filament, or there is a cross connection between the tail light circuit and the brake light circuit, this would cause your problem.
Also, if you have a trailer light harness, check that out as well.