Umm, move everything above 3000Hz down a few dB's? Really, only you know what sounds best in your car.
The most scientific way is to use a real time analyzer (RTA), along with a calibrated microphone, and a pink-noise source. Then you can adjust your system for a nice flat response across the audible spectrum. You could use TrueRTA with a laptop computer, but you still need to get a good microphone.
A cheaper, but less accurate way, is to use a Radio Shack SPL meter. Then you need test tones, one for each band of your EQ. You play each tone, one by one, and adjust the EQ so that the SPL registered on the meter is the same for each frequency. The free version of TrueRTA can be used as a tone generator, with a laptop computer, if your HU has an AUX input, like for an iPod.
Hope this helps!
-- Kevin