first, you have to narrow the problem down to:
1. Is the source material at fault
2. Is your car speakers at fault
3. Is your car amplification / associated electronics at fault.
If you have plenty of amplifier head room and speakers that can withstand the bass, and you are sure that your source material is the best you can find, use an graphical equalizer and apply a 'house curve' to enhance the entire bass range.
Ive done this with just an Sound Blaster Audigy soundcard. Load the file into foobar2k, play around with the equalizer, set the SB recorder to "record what I hear" and trial and error as required.
If your source material is just shoddy in quality, not much you can do on the cheap (time/money-wise).