The fan on my heater only works on setting 4

This problem has been going on since 2nd Dec 2014 and has been to my garage 4 times. They have replaced the resistor 3 times and today took it to a Vauxhall garage where they have had my car most of the day. My garage have told me the dealer is telling him they are unsure what the problem is. It may be the wiring to the resistor which will cost £200 to fix or it could be a new motor costing £300. I cannot believe a dealer who work on these cars all day cannot find the problem. I've been advised to get the wiring done and then it may turn out to be the motor. A payout of approx £500. I have already paid early Dec to have the resistor replaced at a cost of £100 for which I am no further forward. I need some advice on what could be the problem.