It's the resistor pack, reason why it works on 4 is as this has the direct live.

The problem is the pollen filter, people just don't fit these properly and water gets through into the motor itself siezing it and also dosen't help the resistor packs to fail!

For this reason Ford sell the pollen filter seal, which should come hand in hand with replacing the filter!

How do they KNOW it's the motor, as the switches can fail as well?

You can try freeing the motor IF it is siezed, failing that if you work on your car you could do it yourself and just pay for the new motor and resistor pack, £400 seems a little pricy but then it takes a while to remove it I should think!

Martin