We have a 25 year old Subraru that needs a climate control unit. Our local mechanic has not been able to find one. It is a great old car that still gets 25 mpg but is not fun to drive in the summer or winter.
You may not be able to find the part that you need at this point, since it sounds like this is a 1984 model, or thereabouts. Back then, Subaru did not sell a large number of cars, and–of course–finding new parts for something this old is not likely. Your only possibility is something from an “auto recycler” (junkyard), but even this may not be successful.
I know these cars pretty well, but I’m not really sure what the problem is or what specific part of the control unit is faulty.
If this is an '84, or thereabouts, Subaru then they were prone to a few problems in the control unit; temperature control, mode control, etc.
I’ve repaired more of these than I can remember.
The usual cause was the plastic around the cable cracking which would then cause the outer housing to move rather than the inner cable.
A cable problem can be repaired by the use of a lawnmower cable, which is far superior to what the cars were equipped with when new. Anything else is a toss-up since it’s unclear to me what the exact problem is.
(I’ve even got a stash of old Subaru stuff in my basement so there might be something in that pile that would work.)