My 2004 pontiac montana blows cold air in front and rear only, I drain the air from the cooling line already, but still cold air comes out.

SOURCE: 2004 Pontiac Montana blows cold air in defrost

There is a mix damper inside your duct that controls the heat over heater core and ac evaporator that to your floor and one to your defrost. it is probable an electric actuator, that is not functioning properly. If yu can locate it and manually move it, you might have to remove the actuator motor first, you should be able to figure out your problem, I hope this helps! merry Christmas!

SOURCE: I have a 2005 pontiac Montana. The heat in the

Get the heater core flushed for the front heater,if the valve in the upper heater hose actually moves.

SOURCE: 2002 Pontiac Montana, No Heat, I put the temp control on

the cabel from the heat dial to the heater box behind the dash could be snaped or jamed thus not opperating the lever which operates the flap which open and closes the heat box next to the matrix. or if your coolant is brown in colour it could be your matrix is clogged with sedament and needs flushing out. or it could be your water pump not pumping coolant round your engine or it could be a knackerd thermostat i hope these will help you id start with the easyest first thermostat