This is the problem that led to Vauxhall Zafira B fires. Do not simply replace the resistor pack without first checking there is no fault with the heater fan motor, because if you replace the resistor pack and the motor is faulty it is possible that the resistor could overheat. Since the car is still under warranty, refer it back to the supplying Ford dealer, and write a letter sent by post office special delivery to the dealer principal outlining your worry that there may be a fault with the motor that could lead to a fire. Staple the certificate of posting to a copy of the letter so it becomes a matter of record and that forces the dealer to do a proper job. For an aftermarket warranty, Warranty Direct or Warranty Wise, but take it out before the Ford dealer warranty expires or initial cover could be restricted.