Lets call your old thermostat "thermostat A"...New one added to your system "thermostat B"
On thermostat A look for the G marked terminal, it should have your A/C wire hooked to it....REMOVE the wire going to the G terminal on Thermostat A.....
Next take the wire that is running from thermostat B (W marked one) and attach that to the "now" empty terminal G on thermostat A....wa'la your done---you will have the reverse the 2 wires in the spring for the A/c to work....there's other ways you can do this--but this is the easiest way.
Basically, the G terminal on a thermostat is the fan activation wire----when you do not unhook the A/C wire from thermostat A when you hook up thermostat B to it---the thermostat B wire backfeeds the A/C system and turns it on.
I'm a picture dunce on this forum..sorry