Here are a couple of basic things to check: 1-Make sure the coolant level is correct in your radiator and overflow reservoir. 2-Make certain the thermostat (in the water outlet housing) is in good working order. This should be replaced every 24 months because it is cheap/easy insurance that your car's cooling system will run well. Also, make sure that the thermostat in the car is the correct one (with regard to temp) stated in the manual. 3-Are the hoses leading to the heater core (in the engine bay) swollen/damaged/old? 4-Is the actuator arm/servo/solenoid for the heater core bypass functioning correctly? Basically, when you "turn on" the heater, something has to physically open/close the coolant bypass valve to the heater core to allow warm/hot coolant to flow into the heater core (under your dashboard). If this process is electronically controlled, it is possible that it is failing to open that valve some times. If it is a mechanically controlled process, the heater bypass may be obstructed. 5-Heater core may need replacing. If the heater core has a lot of build-up in it, the coolant will not circulate as easily as it should which can cause the heater to not function properly. There is probably more to check, but its nearly 3am and Im not thinking clear enough to proceed any further. Good luck.
The heater motor speed resister is burned out.
The blower motor speed resistor is burned out.
If a 2000 Grand Am sometimes has the heater blower working and sometimes it won't, the problem could be with the heater relay switch. The heater relay switch lets the heater blower know when to come on and when to go off. In most vehicles this switch is behind the glove compartment. This part sometimes gets corroded from condensation in this area.
Not knowing what year, make and model you have, I would suspect the heater fan switch and or electrical plug to the switch is at fault.
The blower motor speed resister may be burned out.
it is usally a dry connection on the fan speed switch
Probably your thermostat
the heater of your car does not only work when the car is in motion. it only works when the engine is started. you don't have a heater in your car like you do your home. the heat that you feel when you turn on your heater in your car is produced by the engine. and there for if your car is not on then you will not receive any heat. some cars take longer than others to "heat up".
Normally when a heater blower motor only works on the High setting, it is caused by a defective Blower Motor Resistor Pack. The fix is to, Replace it. In very rare instances it can also be a defective blower motor speed control switch, but rarely.
speed cluster only works sometimes
Resistor on the heater blower motor has failed
The heater blower motor speed resister is burned out.