The Nest Thermostat – where does the “C” wire go?

[Dynamo2000]

A customer of mine recently asked me to install a Nest thermostat (not the learning model just the regular model) for their combination electric AC/heating unit.  There were 4 wires connected in the old thermostat: Red, Green, White and Yellow.  I connected those 4 wires to the corresponding terminals in the Nest thermostat.  The Nest thermostat seems to be working OK except it displays an error E298 which indicates that there is no power supply to the thermostat i.e. it is running on the batteries.  To remedy this issue Nest (Google) has a small module called a Power Connector which is supposed to be interconnected between the thermostat and the control board of AC/heating unit.  Attached to this post are two wiring diagrams, one for the Power Connector and the other for the AC/heating unit.  If you look at the wiring diagram for the Power Connector you can see there are 3 wires from the Power Connector which go to the control board of the AC/heating unit.  Two of these wires the Yellow and Red also connect to the thermostat and so they are simply "spliced" into by the Power Connector.  The third wire is labelled C on the Power Connector and C/Com on the control board.  This is where I am having difficulty.  Can anyone identify on the wiring diagram for the AC/heating unit where the C/Com terminal is?  In trying to make this identification (without looking at the wiring diagram) I tested every single terminal on the control board.  Every one of them either had 0, 120 or 240 volts to ground.  Two of the wires from the thermostat don't even terminate on the control board, they simply pass through and go somewhere into the wall.  The two wires from the thermostat that do terminate on the control board both tested 0 V to ground but 24 V to each other! (how does that make sense?)  Can anyone tell me where the C wire is supposed to go?