You'll need something like this -
http://mobile.jlaudio.com/products_cleansw....php?page_id=94
or
http://www.mobileaudiocontrol.com/dept.asp...=17612&l1=17612
Stinger/PIE also has some products you can use and so does Scosche -
http://www.scosche.com/scosche_caraudio.aspx?CategoryID=36
Any of those will work.
I have personally used and installed an Audio Control EQX. When the EQ and Crossovers were set, it was one of the best sounding, yet very simple systems I had ever installed.
That unit is too much for most people, I would suggest one of the simplier units and they cost quite a bit less as well.
Good Luck