You should check the thermostat and the fuse for the heater.
Should be located at the engine end of the upper radiator hose in removable housing
take apart the thermostat housing and pop it out be sure and replace the gasket too. There should be two bolts holding it together.
The thermostat is on the top of the motor where the top radiator hose attaches to it. The housing for it should be held on with two bolts. Remove the bolts and lift up the cover. There's the thermostat.
Drain all the coolant from the radiator. Disconnect the upper radiator hose from the engine. Remove the aluminum thermostat housing. Should be held in place with 2 bolts. There you will find the thermostat.
when should the vapor canister be replace on a 1996 pontiac sunfire
The wiring diagram for the power windows for a Pontiac Montana should be in the owner manual. A manual can be purchased if the original is not with the car.
by the lower intake manifold while you are doing this you should replace the intake manifold gasket anyway because it will leak sooner or later.
Find the upper radiator hose then follow it to where it is clamped to the engine. That mounting is the thermostat cover; remove it and the thermostat is inside. When you replace it make very sure you put the new one in correctly. It should be marked or have a picture of how it faces in the instructions.
Unless the housing is damaged, there is no reason to replace it. Just replace the gasket.
The thermostat housing is dome shaped at the end of the radiator top hose on the block, and should be near the water pump which is on the front of the engine. Not looked at a Montana lay out so can't help further.. :)
The correct size should be 215/70R15