I don't have a Pioneer, but suspect the second AC prep to be very similar or the same as my Sundance.
You should find breakers in the AC panel marked "Front AC" and "Rear AC". In my case, the second AC would be in the bedroom in the front. Before you go poking around, shut off All ( yes I said All) the breakers. Don't rely that they are correctly marked.
You should find the AC prep at the roof air vent. In my case, in the bedroom. Remove the interior trim on the air vent (should be just four screws) and pull down the trim.In the cavity between the ceiling and the roof, you should find a romex for the AC power, a multi conductor cable for the thermostat (brown in my case) and a 12 volt cable. These are the AC prep wires.
For the thermostat, cable, you will have to search for the other end. It should be in a place where you can get access to it without too much trouble. Mine is in the wall next to the bedroom slide switch. Yours may be in a different place. Carefully pull out the light switches or other control switches and look into the wall cavity. You should be able to see it. If that fails, then I would recommend you contact Heartland as Dan suggested and ask where it is supposed to be.