Though every car's acoustics will be different (I drive a Pontiac Bonneville), I find that there's a lot of mid-bass emphasis between 150 and 400 Hz - kind of a "thickness" that makes the bass muddy. I tend to leave everything flat in the lower regions, except that I back off on the controls between 180 and 360Hz by about 3 to 4dB. If vocals tend to sound a bit shrill, it might be worth trying a 1 to 2dB cut in the 1KHz to 2KHz region. (You'll be surprised at how much of a difference a couple of dB makes!) I usually leave highs alone, though I sometimes back off 4.5KHz by 1dB and 9KHz by 2dB if the recording tends to be a bit bright.

For me, these subtle changes have made recordings a lot more listenable and enjoyable without "over-engineering" the sound.

Greg