Just get a used stock fan and its 4 speed resistor/wiring from 86+ air
conditioning. I pull them from junked vans for this purpose, only if the
A/C ceiling unit rear fins are pretty clean showing little use.

The same fan switch comes in a 4 speed version for the same price as our
3 speed switches. Fits the same, works the same. Add a wiring pin to the
harness connector for the extra speed.

No need to pay the pirate from chicago his ransom for a resistor either.

Mark

Alistair Bell wrote:
 >.......But apart from that, a PWM controller might be an interesting
  >  project - liberating us from the restriction of *only* three fan
  > speeds :)
>
> alistair
>
>