I think it will be the resistor thats behind the glovebox, you pull glovebox out the way and you can see the fan motor housing and its on righthand side, think it will be this part

http://www.ebay.co.uk/itm/1208138334...ht_5787wt_1155

Question is why is it playing up, have you had a new screen or the pollen filter changed? They let water in and get onto the fan motor and relay and cause problems, fan motor is about £30-40.

Hope that helps