2005 Dodge Ram 1500 - The AC compressor only engages and cools when in defrost mode but not when in cool mode?

When dialed to defrost mode with the temp dial turned to cool, the compressor engages and the cab enjoys SOME cooling, however, when the dial is set to cool mode, the compressor does not engage and hot air is blown throughout the cab. I am therefore uming it is a switch, but what switch and where? Hellllp it's 94 degrees here today and I have a 5 hour road trip in my immediate future! Thanks!