When I installed and someone wanted to keep their factory head unit but add and ampifier we used one of these
http://www.techforless.com/cgi-bin/t...20100504023701 They're pretty easy to hook up. Basically you tie into your rear speakers (either form the back of the HU or in the trunk directly from your rear speakers) and it takes a high level from your speakers and converts it to a line level for RCA inputs to go to the amplifier. As for the aftermarket chime...you can do it, just keep in mind that you will lose the chime so if you leave your lights, key in the ignition, stuff like that it won't "warn" you.