For those that don't know, modern cars pulse the compressor - if you're sitting idling, you can see the revs rise and fall slightly as it does (about every 10-seconds usually). Also, if you use more than about half throttle, it cuts the AC so you get all the goodness.

Not as much charm as that but my Volvo 240dl had various electricals that broke & I'd just run a new line, to a big chonky metal surplus toggle switch on the dash. Folks seemed to really love it!