I'm installing a new thermostat. The old one just controlled heat, and my idiot brother-in-law (who works for trane btw) somehow rigged it to work the heat and air. The wiring for the heat was pretty self explanitory, but the wires to the air are not labeled and I have no idea which is which. Can someone help me identify these 2 wires?

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

The furnace can't tell what color of wire is connected to it. To make the air conditioner work, you must have a wire running from the Y terminal on the thermostat to one terminal on the coil of the relay outside on the compressor unit. The other terminal must be connected to the other side of the transformer. Follow the wires that are now connected to the compressor. They may be 2 of more in a cable. One should connect into the furnace transformer. The other should run to the old thermostat. Just connect it to the Y terminal in the new thermostat. The wires may go to terminals in the furnace labeled Y and B at the furnace.