check out the headphone thread I just posted I was going through the same issue.
testing tracks from an mp3 plyer into the aux and hearing the mud.
this beyer dynamic virtual studio plugin has a car emulation that is really close. other than some extra reverb, it nails what frequences are boosted/cut.
I was able to get my bassline sounding way nicer and discovered i shoulda cut out a bit of lows in a kick
i cant post up links yet but search for it at the jrrshop.
best part is its free, and works for mac or pc
theres another one called Isone too, also free
should save us many trips to and from the car and all those extra miles