how can i fix the fan switch on my astra?

p reg vauxhall astra. The first 2 positions on the heater blower switch stopped working but positions 3 & 4 still worked. now pos 3 has also packed up, only 4 still works. I'm guessing that each position has a different value resistor across the supply to the fan, so to fix it i need to replace the burnt out resistors.
Am i guessing right or is a different system being used.
Any help appreciated.