The three most common problems for heater fans are
- Burned-out blower motor resistor. This is usually located on the engine side of the firewall. Sometimes one section of the resistor is burned out but others are still good, so the blower only works on the highest speed settings. If you have a burned-out resistor, check the blower bearings to make sure they are not stiff and causing the motor to draw too much current.
- The blower is actually working, but either the intake vent is plugged (I've seen vehicles with the heater core completely blocked by dirt, leaves and insects that came in through the vent), or the heater door is stuck closed.
- The motor bearings have gone bad (see # 1). This can cause the fuse to blow, although the resistor usually fries first.
I'm not sure how your car's cabin environmental control system is fused. Check any fuse that might apply, such as Vent/AC, HTR, FAN, etc. Generally you can identify the state of a blade-type fuse by inspection without removing it. If you have a blown fuse, check the blower motor before wasting money on a new fuse that will probably blow. Like any fan, it's supposed to turn easily, and there should not be any roughness or side-slop in the bearings.