If the oem head unit has cd changer controls (it probably does), just add the oem changer. This isn't the most cost effective, but is the easiest, you can install it yourself, I'm sure. Your other options...
-Buy a Blitzsafe adaptor, which will adapt your oem pre-run changer DIN cable to an aftermarket connection. You will need to do a little digging and find out who makes the factory head unit (Alpine, Clarion, etc.)
-Replace your oem head unit with an aftermarket deck and changer