It's possible the issue is in the wiring or the climate control itself.
Neither of these is a high frequency failure and the climate control head
is rather expensive. I'd therefore recommend spending the time to check
the wiring thoroughly.

Unplug the power transistor. Check to see if you have voltage present in
the Black/Orange wire. It may be somewhat reduced from 12V (depending on
your testing device) because it goes through the blower windings prior to
arriving at the power transistor.

Check for ground in the black wire. You already know the connection at
the ground location is good because the fan works on high speed and this
uses the same ground lug. Your voltage between the black/orange wire and
this wire should be very close to the same thing you got in the above
test.

Check for continuity in the Green/Black wire. You'll need to remove and
unplug the control head and check from one end to the other. Also check
to make sure this wire isn't shorted to ground (i.e. check the resistance
between the wire and a verified ground and verify it's an open circuit).

If these three wiring tests check out and the power transistor is known to
be good, then you have a defective control head.