The ACC II systems use a Darlington pair 30 amp power transistor in the fan controller.  ACC I systems use power resistors.   If the fan sticks on high speed, usually a shorted power transistor or an open resistor is indicated, depending on which system is in place.  Another failure ,mode for the transistor is erratic blower motor operation, although I have also experienced the same behavior with a bad blower motor.  This write-up is for the ACC II.

Things that can make the transistor fail: Insulation comes loose from the inside of the evaporator, gets into the fan and the excess load blows the transistor.  Also, excess load due to water in the fan rotor cage (because of clogged evaporator drains).

Be advised that the heat sink for the transistor is at +12 volts when the circuit is in use, so be careful not to let the heat sink contact any chassis metal when the circuit is powered.  Normally it is inside the plastic evaporator housing, so it'd no problem.  However, if you actuate the system while it's removed from the housing, you need to remember not to contact anything metal chassis with the heatsink.

The ACC II fan controller is in the front of the evaporator housing on earlier models (my 91, for sure).  On the 94, it's closer to the blower motor, more toward the top of the housing.  Pictur (of the 91).  To get to it on either the 91 or later cars, you have to remove the aquarium cover, then remove the false bulkhead.  Remove the four T10 Torx screws that hold the controller in.  Disconnect the electrical connector and remove the unit.  By the way, while you have the unit out, reach inside the evaporator housing and find all the loose insulation you can.  Dry everything out thoroughly and use an aggressive rubber cement to glue the insulation back in place.

As for the ACC II system's transistor, the Toshiba part number is 2SD1525.  Radio Shack used to be able to supply a replacement, but that is no longer true.  Many thanks to Glen McNeill for providing these alternate sources for the Toshiba part: Findchips.com is a good place to look for parts suppliers. Go there and enter 2SD1525. It will tell you that Newark Electronics has them, and will provide a link to their site. I assume us average one-sie two-sie consumers can purchase on line from Newark, but I'm not sure.  Glen got his from Audi Lab in Georgia. Go to the Audio Lab Web site. This should take you directly to the shopping cart page for the transistor.  As of 11/94, cost is around $9.00; shipping is nearly as much at $8.00 or so.  While you're at it, order some heat sink compound (grease): http://www.audiolabga.com/mal_cart/mal_cart.php?find=10-8109 (or just search for 10-8109).

Here's another possibility, provided by someone on the Saab bulletin board: http://search.mediagrif.com/bfparts2?/WebWizard/List/MemberID=SUMMITEL

And another, provided by Brian Spencer:
http://www.mcmelectronics.com     $9.10 for the transistor, he says.

If you didn't order some, go to Radio Shack to pick up a small tube of heat sink compound (grease).  You coat the back of the transistor with just a thin layer of it so the transistor can transfer its heat to the heat sink; it's important.  This stuff is really messy, as you will find out!

Pry off the cover.  Remove the screw that secures the transistor to the heat sink.  Remove the screw that secures the board.  You can now pry the board out of the housing (for the earlier cars, you'll have to release the retaining clips on each side as you pry it out; for the later models, you'll need to pry the connector loose from the housing - might be true of the earlier ones as well).  Note the orientation of the transistor, then unsolder the old transistor, then bend the new transistor's leads to match the original.  Solder in the new (you'll need a reasonably powerful iron).  Before reassembly, smear a thin coat of heat-conductive (heat sink) grease on the back surface of the transistor.  Picture  Slide the board into the case, secure the transistor with its screw, secure the board with its screw, then reinstall the cover.  If you test the controller before reinstalling it, be absolutely certain that the heat sink doesn't contact any part of the metal chassis of the car (the heat sink is at + 12 volts when the unit is operating, not at ground).