I think it's pretty comon it happened on my mk4 too. Basically on the mk4, the resitors on the circuit board failed. Small component, big fook up. Anyway so what happens when the car gets a little warm, it turns on the big boys. Not the small fan. New units are required due to the intricate placement of the resistors. Ironically this problem was caused by excess heat. The resistors really need a seperate heatsink to disipate the heat better. One can only assume the same thing can happen on the mk5s