The resistor is the likely cause and worth replacing in the first instance, it's located on the heater box and accessed from the passenger footwell. However i had a similar job and this cured it but a couple of weeks later it happened again due to the fan motor being faulty? Replaced both items and all cured.
Jimmy
March 2012
heater blowers usually have whats called a resistor pack.a series of resistors and depending on what speed you want which resistor is used.they can burn out and cause your problem.usually by or in the fan unit behind the dash.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.