Hey Bill, why not get a pair of monitor headphones to eliminate, or at least reduce the need of "car-testing"?
always need to car test. always need to test the mix in as many different locations as possible, and the car is a particularly unique location and it really allows the song to be heard from the listener's perspective, especially because you can't make any changes while in the car. my car system is actually not too good and it's a bit strange and probably has too much bass, but it doesn't matter. it's still important to test it there