Necro'ing my own thread, but I fixed it in the end. The manual heater control knob is directly connected to a rotary knob somewhere under the dash. As you turn the knob, you turn a shaft that turns the knob under the dash. The shaft can get disconnected if you pull the heater unit. The knob under the dash can be reached from the foot well. I ended up getting a helping hand (credit to the gf) to feed the shaft through the dash panel, down to the foot well so I could connect it to the rotary switch. Afterwards I connected the half-connected shaft to the back of the heater control unit and popped it back into the dashboard. Car was fixed, happy ending.