2000 Mercury Cougar revs up when I turn the heater (defogger) on and makes shifting a little harder

If moving the HVAC control on the dash makes the RPM increase in AC or DEFROST mode and the signal goes to a blown or otherwise defective compressor, it will also send a command to the ISC motor to increase the engine speed to help compensate for the added horsepower loss due to a good compressor's drag. However, 2-2500 RPM sounds a little high. Again, anticipation of the power loss of a good, working compressor, also increases the parameters of the transmission operation on most computer controlled systems. If your compressor isn't engaging b/o mechanical failure, loss offreon, etc, the computer has insufficient information to make the correct performance changes. Years 2000 and below weren't the best time for computer intelligence in the automotive world. This may not be the whole problem, but it's a start.....not being there to actually see the full picture makes it hard to diagnose.