Go back and read my first reply.
I will add to it here. The servo is controlled by the amplifier. It does this by powering the small DC motor in the servo. The amplifier changes polarity to move toward heat as opposed to cold.
The amp is a comparitor. It takes the sum of inputs and compares what exists to what is being asked for. The decision is implemented by winding the servo one direction or the other. The various inputs are: incar temp sensor, ambient air temp sensor, evap temp sensor (?), servo feedback potentiometer (tells what position servois in and shows movement to amp) and the temp wheel setting. Vacuum is an output from the servo. The only input to the servo is the two wires that power the internal motor.
The two vacuum lines at the bottom of the servo are important. Unlike the massive group at the top of the servo, the are an input of a sort. They are at the beginning of everything and pass through a temp switch in the water part of the servo. By not passing vacuum at low water temps the master vacuum switch will keep the blower from running. I forget the temp at which it occurs but I don't think that temp exists in Florida as I have never seen it operate. In normal op conditions vacuum will be on both sides. Since you have blower forget about those lines. And, YES the defrost position should over ride everything. The servo should move.
Without a diagram and my neat tool I can't say whether the servo is controlled by the amp during defrost. I think so as I don't know where the power would come from. I'm sorry but that doesn't eliminate the amp from consideration.
I would identify the two wires going to the internal motor and check power. The voltage polarity should reverse by moving the temp wheel from hot to cold. If you can't figure it out from the diagram, take the top off the servo; its easy. There are four screws under daps of black silicone at the four corners on top.
Bosch Master, ASE Master, L1
33 years MB technician