This is a shocking answer but one of the heaters may have a break in it's heating element liner and be the culprit. You have to cut the AC power to the heater and then remove both sides of the electrical connections on the heater elements. ( Most times there are two) with a ohmmeter there should be no connection or leakage from the heater element electrical terminials to the tank metal
Replace fuse #36 in the inside fusebox.See "Related Questions" below for more
Ck the heater fan motor resistor .
in the dash but check the resistor 1st the blow e.z. it is in engin comp. passinger side on heater core lines it looks just like a water heater fuse
short circuit in the circuit
it can be caused by a bad alternator
If the heater fails to "blow," then the fan motor (situated on the fire wall inside the engine compartment) is disconnected or has failed and should be reconnected or replaced. But, before either of those, check the heater fan fuse, replace if blown, and re-try the heater.
One of your wires that have to do with the Reverse light are touching a another wire that they shouldent be touching ( + is touching a - ) it makes the fuse blow...if any changes made recently with any wires rewire or check if correctly wired
If the fan is not blowing, a fuse may be the cause. If there is just not warm air being blown by the fan, check your coolant level in your radiator. Low level of water and coolant will cause the heater to blow cooler air.
in the water heater
Defective blower motor, blown fuse, or a defective Blower Motor Resistor Pack.
it could be your coil pack or a short
have you checked your fuse? If it won't blow out the air it could be your blower motor.