Hot rear seat in Dodge Grand Caravan '06

In interstate driving this minivan has developed a peculiarly very hot seat, second row, behind the passenger (front) seat. It is so hot as to make it uncomfortable for anyone in the passenger seat, without AC on and outside air intake – and impossible for that seat to be used. Seat behind driver’s seat is fine. Dodge dealer said nothing wrong; independent shop replaced exhaust system, no change… Help?

I don’t think the rear seats have heating coils in them like the front seats can unless that has been added in later years. There may be a heating vent under the front seat that isn’t closing off when it should be.