the rear heat does not work on my 2002 Pontiac Montana?

Aero D good fail on answering that question. that's part of the reason i stopped answering questions 3 years ago, and I see people are still peddling their jibberish. The resistor gets hot and the blower blows that heat? By the way, the fluid (coolant and water mix) is where you get your heat from, a car is not a baseboard heater out of a house. You're partially right on the blend door problem - the temp is the issue, not the ability to get the air itself. Whether you have manual or digital control, the heater control valve or blend door has to be moved in order to get the flow of coolant into the heater core, and the hot air to get past the door. You either have a stuck door or more likely your heater control unit in the dash has defective electronics. Try a junkyard assembly on the switch first, if that doesn't work, teardown of the ducting equipment including blend door may be needed.

There isn't any water flowing for the heat, in any vehicle.. Might be a bad blower motor resistor. THe resistor gets hot, by resistance, and the blower blows the heat around..

Good Luck!