The switch is probably OK. You'll have to replace the rheostat on the motor which is accessible under the glove box. This has to be dismantled first and is a fiddly but do-able job.
Walter
March 2010
you need to change your heater element, behind glove compartment, held in by two screws,on top off air box lossen screws first then take off 4 pin plug you can get part at 24/7 spares £5 this will fix your problem
ps
clove compartment is held on by 2 8mm bolts, bottom right/left corners
dean morson
January 2010
it's usually the resistor pack. it can be found attached to the motor blower box which is behind the glove box. go to www.rezpax.com . for more details & pictures
jim
January 2010
I can answer this question...
my rover 45 heater fan only runs on two speeds how can i fix it?