This doesn't matter if you have one or two A/C units, but the phone type cable will go from the thermostat to the rear A/C unit, probably your rear unit will be a 15K A/C. The cable goes to the control box mounted just above the filter, and possibly you might have corrosion which will prevent power getting to your thermostat. To access the box, you will probably have to remove the entire ceiling filter cover. We had corrosion a number of years ago in our front A/C control box and I replaced it. The photo is of the circuit board, which has the jacks that the phone type cables plug into, at the top of the photo. If you have a second A/C, you should see two cables plugged into those jacks, one from the thermostat and the other going to the front A/C. There will be a box under each A/C unit. There will also be wiring to that box from the furnace. This is the box that the thermostat talks to:
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