In car audio systems with active feedback built in, the sensor mikes are usually built in (in pairs) in the driver's headrest.

Notice that this thread is not about noise cancelation (which we've discussed at length and the consensus was that it's way too hard to do in a random vehicle) but about something much cruder: increasing the volume as road noise increases (and maybe boosting the bass?). This means that you don't actually want any feedback from the player, or it'll just play louder and louder until your speakers blow up (or your eardrums).

Inside the main compartment or the trunk/boot it'll be getting feedback from the speakers. In the engine compartment is probably no good either, engine noise would vary too much when you step on the gas and is probably not a good measure of "road noise". I was thinking that the mike should ideally be outside the vehicle but still be protected somehow. Hidden under the rear bumper? Rain will still get to it. Something along those lines, I still haven't found the perfect spot.

Borislav