Heater or defrost? Defrost engages your A/C compressor to keep the system from drying out during winther months. If the A/C is indeed coming on then you may have a problem with the compressor or line freeze. High pressure will stop the compressor from turning but still engage the clutch so the engine is trying to turn everything. The belt loses in this case!
Easy to check, pop the hood and turn on the heater. If the A/C clutch engages that is most likely your problem.