Fan only works on high speed

If you are talking about the early 99 which is the MK3 style then I can point to the switch that you are turning on the dash. Poor design which will melt the contacts and cause overheating. You can remove the button by pushing a tab on the bottom and look at the contacts. Turn the switch and push on the copper contacts and see if you get any other speed. Watch out the contacts get hot. The other choice would be the resistor pack located in the heater assembly behind the glove box.