Why didn't I think of that before!!! My wife's Jeep Cherokee doesn't have an A/C button like my Honda. I'm going to do this on her Jeep NEXT winter... reason being that her A/C just stopped cooling I hate A/C Repairs!!!
A lot of vehicles with separate buttons for the a/c still turn on the compressor for defrost. My truck does it. One thing to think of though... once a month plug it in and run the compressor for a few minutes. You need to keep the oil distributed throughout the a/c system to keep the parts lubricated, protected, and to keep the seals from drying out.
Are you sure this is the best idea? You want the compressor running in the winter to dry the air out inside the car. The greater the temperature difference between the window and the inside of the car the less humidity it takes to fog the windows. I run the heater without the ac quite often but at least once I have to turn the compressor on for a minute or two to get stuff off the windows. ESPECIALLY if it's cold and rainy and i got into the car wet.
It's true removing moisture is a good thing, but as long as theirs air flowing over the window it will prevent condensation unless you have a major humidity. And if that happens just open your window.
I really hate how all new cars turn it on by default and most don't even give you the option of turning it off, if you defrost you use AC end of story. Pulling the relay is a good idea. That might just trick out the girl friend, she'll be suprised next time she tries to use the AC when it's 55F out :/.
The only ones that didn't turn it on by default that my family ever had were Chrysler K cars. Everything else turned the a/c on. The defogger works better with the a/c on and most people out there aren't smart enough to kick it on to defog the windows when needed, so the mfr's just make it automatic. On my truck when defrost is selected it does not activate the a/c indicator light either. I'm sure that was done to eliminate complaints. "Its 40 degrees outside and my air conditioner won't shut off". GM loves automatic stuff. I remember on my old 74 Chevy truck when a/c MAX was selected the blower automatically kicked into high speed, no matter what speed was selected on the switch. I promptly disabled that. It would get far too cold, too fast. It would literally give you a headache like you just chugged a milkshake. Man that truck had the best a/c ever.
The A/C only runs with the fan on so if you keep it on defrost and not run the fan - just let the air flow naturally as you are driving it will not run the compressor. Yeah it does suck that it runs on defrost and I really notice the major hesitation in the engine between shifts when the A/C compressor is running.