A/C woes

I recently R&Rd my dash and heater core box to replace a leaking heater core. After reassembly, the A/C no longer works; it only blows hot, and the ambient temperature reading on the climate control unit reads 45F regardless of the outside temperature. I swapped both a good spare head unit and a good spare ambient temperature sensor into the car, and still got readings of 45F.
I followed the ambient temperature sensor/circuit analysis according to the FSM. With the circuit open, the dash reading should be -56F, which is what I got. With the circuit shorted, the dash reading should be 181F. I got -13F. What I interpret from this is that I have neither an open circuit nor a short. Does anyone understand what this is telling me, or have any suggestions how to fix this problem?
All the vacuum lines are good and correctly reconnected.