check the wireing harnesses
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.
The blower may simply have burned out. If the blower is out then the heater probably still works.
Bad blower resistor?
Probably the blower motor speed resistor is partially burned out.
it is either a bad switch on your controls, or your blower motor is going out
Have you checked your coolant? Kind of obvious but sometimes overlooked.
The blower motor speed resistor is burned out.
The most likely cause is a bad blower motor resistor.
Depends on what you mean by heater will not work. If the blower motor will not work, it can be a blown fuse, bad blower motor, bad blower motor switch, or a defective blower motor resistor pack. If you mean that the blower motor works but there is no heat, then it can be a clogged heater core, thermostat stuck open, or defective heater control valve.
try wiggling the wires at the blower resistor if don't work then tap on blower motor