Went to a job yesterday where one of my coworkers removed a blower assembly on a Luxaire 90% efficient furnace. My job was to replace the blower wheel and get the furnace back together and running. As with nearly any furnace newer than 20 years, all the wires to the thermostat, humidifier, and the air conditioning condenser hook up to a control board IN the blower compartment. My problem was that the wires to the condenser and humidifier are exactly alike. Both are small and have a red and a white wire. So how do you tell? The wires should have been labeled, but the trick to finding the correct wire is with an ohmmeter and with the humidifier shut off (if it's a manually controlled one, or unhook them from the humidistat). Ohm out the two wires, and the ones that register zero are probably the ones to the condenser, since these energize the coil to the contactor.
This will work for about 99% of the air-conditioning systems out there. If the unit has a control board outside, or the humidifier control is electronic, this may not work. Better to just label the wires as you take something apart. Maranatha!