Get a multimeter and check you are getting 12v into the switch to begin with (with the ignition on).
If not, trace the wiring back to the fusebox and check the fusebox itself - they're absolutely rubbish on the 240 and get covered in rust. Even when they're clean they're not totally secure, and the bullet style fuses have a tendency to start melting their own insulation with the slightest bit of heat which makes things worse.
Got 12V to the switch? Move towards the motor. Resistor pack may or may not be worth checking. On full power it shouldn't be going through any resistors, so I'd be more inclined to look for dodgy connections or wiring.
On the motor itself, they rarely fail outright, more likely is seized bearings which give plenty of warning in the form of screeching before they start causing issues.
If you pull the stereo out and pull the ducting and mesh out from behind it, you can get your hand in to see if the fan blades move freely.
If they do, you should be able to see the wiring coming down the side of the motor. Either tap 12V in there, or follow the wiring back to a more convenient place and do the same.
If it spins freely by hand but doesn't work, it may need to come out so you can look inside the motor and check the commutator and brushes. Usually it'll be the commutator needing a good clean if it has been standing a long time, or if a brush has cracked or broken up it should be straightfoward to replace.
Getting the motor out.. that's a different story!
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