I was thinking about just using a ballest resistor for low speed. Safe and works well .... okay need to test if it works but should do the job. Thanks for the advice.
Get a 4 pin. You can wire it in different combinations to get the speed you want......
1....high resistance side only
2....low resistance side only
3....wire both in parallel will give you lowest resistance
4....wire both in series will give you most resistance
Be sure to wire it so it gets cooling and is away from combustibles they will run HOT!!!
I was thinking that the defroster mounted the resistor in the airstream, somehow, maybe the intake to the blower?
