Never done it. Like Absinth says... Wind the window down. It'd be nice to have A/C on those killer days, but half the fun is watching the fuel gauge go down and the temp gauge go up...
In saying that, I did grab an original under dash unit from an early HQ and am selling it if anyone is interested? Just the unit from in the car, nothing from under the bonnet as it was all 6 and I have an 8.