Well lift the hood and check if it smells in there. The heater: The blower motor can be overheating. It could have some shorted windings. If it gets worse on Hi it's the motor. The resistor is not used on Hi. If it gets worse on low or medium it could be the blower resistor which controls fan speed.
If it burns out try to get the whole blower assembly from the junk yard. Parts stores carry the resistors.
It could be that the spark plug or the coil wire is making contact with the hot engine. Also the belt tension could be incorrect making the belts to slip. You should have your car checked out by a mechanic.