Just picking up on this thread, so after trying myself to diagnose and failing miserably, I spent a few hours on it today with an actual capable mechanic and we think it could be the AC amplifier not functioning (part 88650-14270, though maybe different on my J-Spec?).
I was super confused with the system as a whole, not realising that the ac amplifier and dash control unit was actually 2 different things (even though it definitely says that above, I just didn't quite get it), the amplifier being presumably hidden somewhere in the depths of the dashboard.
I believe the dash control unit has been replaced in the past as it has a part number written on it in tipp-ex (seems like a replacement used part), I've changed the blower motor relay and internal temp sensor, we confirmed today that the blower does indeed work if we send voltage to it so presumably the only option left is the AC amplifier unless anyone else has ideas?
Does anyone know of one for sale/where I could get one? I'll try toyota tomorrow but not holding out much hope, will also try Garage D.