I have Japanese vehicles and recently i had to replace an AC blower squirrel cage affair (Nissan) cuz the bearing failed. The unit had a built in PWM controller with 6, K305, MOSFETS on a small low profile finned heat sink (1" x 3" with 8, 0.25" tall , 0.125" spaced fins) cooled by the same blower outlet. I salvaged the PWM unit as I got a complete blower and PWM as a single scrapyard item for $40. The circuit used two 15A fuses, seems the motor had 2 coils, 3 connections, when the motor jammed it took out both 15A mini spades.
If your motor has multiple coils u can use switches to activate them and obtain different speeds.
PWM for fan motor automotive use is fairly efficient and reliable:
**broken link removed**
I don't see a link to a DIY circuit though. Once u can cool the circuit with the A/C blower it's more than doable with a TL494 PWM controller and a couple beefy mosfets like IRF3206 units.
It is probably also possible to locate the stock resistor in the A/C blower path to prevent it failing, or if you can get scrap resistors, gang two parallel pairs of stock units in series to get 4x power handling and you're done.