I didn't ask if the rear heater was the factory type or aftermarket?
That control pod you have was almost certainly added after manufacture as there were/are two versions; with and without rear control. When in Rear Control position the rear blower speed is set by those controls, it not being adjustable from the front while in that mode.
Its good you'll have your schematics----those are invaluable for all the factory wiring.
Hope you get this sorted out easily and cheaply too.