The problem with mount a mike outside is that it doesn't take into account the (huge) difference between windows up and windows down.

The absolute difference you can adjust for manually when you roll the windows up or down. That still leaves the adjustment speed - I suspect that ideally it should be more sensitive with the windows down. I don't know, you can either make a compromise that works relatively well in both scenarios or you can use a non-linear function (i.e. if the volume is already high then you probably have the windows down, so make bigger adjustments). Hmm, finetuning this might be more difficult than coming up with the perfect equalizer setting.

Borislav