The van does not have any heat. The fan/vent and air conditioner work great. But if I have it on vent, and I turn the temperature dial on the counsel towards hot, nothing happens. The air temperature coming out of the vents remains the same, cool. Car does not over heat, runs great, heater worked great last year. First time it got cold this year and I put it heat, turned the knob t warm and nothing happens. Advice?