Sounds like a couple of resistors have blown on the blower motor.
An auto electrician should be ablet to sort you out if your not inclined to fix it yourself.
I think you will find it is a thing called a voltage regulator.
Same thing happened on my Astra. On the Astra it is not located with the blower motor, but was under the air intakes under the windscreen, and was quite a fiddly job to remove and replace.
it is called a heater resistor and will be located in the air inlet side of the heating system if i remember rightly under the scuttle panel (wiper linkage) this is to keep it cool due to the temp it gets to