The controller has a power FET, some related circuitry and a relay. The relay is used for the max speed. If the blower is always spinning at that speed then the relay inside the controller may be stuck.
Make sure the DATC isn't always keeping the relay engaged. Trace the wiring using the appropriate wiring manual and probe the line. If it is, then the DATC is bad.
Try tapping the controller with a soft hammer - softly. That may disengage the relay. It's tucked well up in the passenger footwell right next to the center console. Black rectangle about 2" x 4" with wiring harness coming down towards the floor.
In a Gen 1, I'm quite certain it can be removed without taking out the dash. It is NOT EASY and should be practiced at a junkyard. I've pulled about 7 for a side experiment and got pretty good at it. Drop the blower motor (Easy), remove all the bolts for the lower blower housing (not so easy, several are hidden), unbolt the controller and wiggle it out while pulling down on the housing. The heatsink is very large, so getting up in there and cutting it off makes removal very easy. You can also slightly trim down the heatsink on the new one to make it go in easier.
Is this better than removing the dash? Well, it's probably 4 hours of work as opposed to a full day or two, but it's pretty easy to cause irreparable damage.
If it's just that the motor continues to spin slowly then I would suspect the DATC more than the controller. There's a separate wire that controls the lower speeds. Probe it when the motor is supposed to be off. If it has any voltage then the DATC is keeping the blower going. If it's at ground, then the controller is damaged. I think that's an odd failure case, but the failures I've caused are catastrophic.
Replacing the DATC is really easy, so if you can get one for cheap then that might be an easy test. If that's not it then you're facing the excitement of the controller.
