Well you can go to either a local car stereo shop, I think radio shack, or best buy what ever is around you, and ask for a PAC SNI-35 here is a link to an e-bay auction to get a look at it http://cgi.ebay.com/PAC-SNI-35-2-CHA...QQcmdZViewItem


I added an amp and sub to my girls car, basically what I did was found the rear speaker wires spliced them together with the front speakers and put the adapter straight to the rear left and right wires, went from adapter to amp to a 10" fosgate and it pounds nicely. Granted you will loose fader controls unless you want to put all front speakers on the left front, and rear speakers to the front right then your balance will act like a fader,
hopefully this helps