it is either a bad switch on your controls, or your blower motor is going 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.
Probably your thermostat
I use V-Power (99), works just fine
Loose connection or blower motor? Bad blower motor? Loose fuse? Bad fan switch?
check the wireing harnesses
Get the air out of your heater core.
The heater motor speed resister is burned out.
BY CONVEntional current
replace front heater corefront and rear have independent heater cores
Possibly
heater hoses might have been disconected