The blower motor and AC works but the heater doesn't in a 2002 bonnieville Pontiac. I turn on the AC and it works fine. I switch over to the heat and it just blows cold air.

If your vehicle has the electronic climate control, it is probably the blend door motor/actuator that has failed and is stuck at the temp that was last entered in the climate control unit when it failed. this can be a very time consuming repair to do as it is under the dash. the other possibility is your coolant level is low and not allowing the heater core to bring the outside air up to the temp you want. if that is the case,your low coolant temp sensor should have come on. if not, then the thermostat could be either stuck open or closed. if stuck open the engine would not always show the temp gauge at normal operating temp of 195-200 degrees. if stuck closed the engine should be overheating according to the temp gauge. you did not mention what the operating coolant temp was!

Possiable you have a bad acuator in the dash