2003 Grand Prix not blowing hot air. Does the heater coil need to be replaced? If so where is it located?

If the heater coil was bad, it would be leaking radiator fluid... probably into your dash/floorboard (depending on where it is installed in your model car). There is a switch that diverts the hot radiator fluid through the heater coil when you want heat. There is also an air diverter valve that helps to mix the hot and cold air. The fluid valve is controlled by the A/C settings control. The one that you use to choose between heater/air conditioner/vent/ defrost. The air diverter is controlled by the temp mixing controller. The most simple explanation is that one of those valves or the controller of one of those valves is not working. If the hot water cant be diverted through the heater coil, the coil can't exchange the heat (warming the air). If the air diverter (hot/cold controller) cant move to add heat, then all you will feel is air from the vent side (from outside) or from the A/C cooling the air.

I would guess that its either a controller or one of the valves themselves. Odds are, it is a controller. That tends to be what malfunctions first.

Heater coils generally don't need replaced unless they are leaking coolant.....usually an engine thermostat stuck open will cause the heater to not blow cold air,,,,Also their are Vacuum devices in the hose assemblies that could be the problem.....check it out online....