I am thinking about changing thermostats for my ICP END4X36T17A2 furnace, but my options are pretty limited due to my current thermostat hookup lacking a C wire. It currently has four wires hooked up to the Y, G, W, and Rh terminals. I followed the wire bundle back to the furnace and looked at how it's connected to the circuit board. I see five terminals there labeled G, C, W, Y, and R; all of them except C are currently running to the thermostat. (There is a separate two-wire bundle running from C and Y to my external air conditioner) It would seem to be a pretty straightforward task of running a wire from the C terminal to my thermostat, except checking with a multimeter I don't detect the expected 24VAC from it. I'm guessing this means it wouldn't work. There are also two tabs next to the Y and R terminals labeled "TEST/TWIN" and "24VAC", but I don't detect any AC voltage from them either. Is there a way to make this work? Is this a job for a professional?

Here is a close up of the terminals (click to enlarge). The four wires running off the right edge are the bundle going to my thermostat.