[north]

ATC must do three things if you think of it:
control air temp flowing into car
control fan speed
direct air to different vents

Most ATC's I've run across do at least some of these. The most challenging one is directing to different vents.

If the ATC can't control fan speed hide a fan switch under the dash and have it run directly to the blower motor.

If it can't control temp you can easily install a non air style temp cable and jury rig it to the temp door lever on the heater box in the car.

If it can't direct air to the right vents you have a bigger problem.

If you think about how ATC operates the most likely problem is the programmer and they surprisingly turn up NOS on eBay. They were so expensive in the day that any that weren't sold by the end of warranty were probably never sold. So there are many out there, especially the later years as they became more commonly installed.

I'm talking big, we shall see, my Bonneville system works fine except for it refusing to go to higher fan speeds when needed. I have an NOS fan control switch in case it's the switch but I'll start in the spring by testing power to the fan etc.

My GP system is an unknown since my fan motor is burned out, I bought a new one which I'll install over the winter and then see from there.