If the blower speed is sporadic, then the fault is at the heater control panel.
You could try using a soldering iron and heat up the solder in the contacts. If this does not fix the problem, you can replace the potentiometer. Or get another second hand unit.
See here:
http://forums.swedespeed.com/showthread.php?70699
and here:
http://www.30forty50.com/forum/topic..._1#entry130968