Just another C wire problem
Run a new cable with more wires.  Then at the air handler connect them as follows:


Red to Rc
Yellow to Y
Green to G
Blue to C


If there's already a wire attached to the C terminal, don't worry.  Just attach the blue wire from your thermostat cable there as well.  The other wire is likely from the condensing unit outside.  There may also be a wire on the Y terminal, which also likely goes out to the condensing unit.

At the thermostat, connect then wires as follows:


Red to Rc
Yellow to Y
Green to G
Blue to C


You should be all set.