[Paul Clifton]

Hi Matt,
from experience I would say you have a problem with the fan resistor pack. This device cuts the voltage to the motor to give you the lower speeds.Fastest speed is 12volts to the motor. All current for the fan goes through a set of points within the resistor pack, on fast it also goes thrugh a relat (the one you can hear under the dash.
The resistor pack is located within the air box under the bonnet, you have to remove the large self tappers that hold the air box to the car (under the bonnet)and remove the clips that hold the two halves together you will then see the device screwed inside. Top tip, bite the bullett and get a new one from Volvo dealer.
Paul.