2004 mustang, change radio only

I need some info on a 2004 mustang Convertible. I want to change just the radio. I got the harness adapter for the main plug. But I noticed there is another smaller plug. when I unplug the small harness it cuts out the 2 bigger speakers in the back. Is there an OEM amp in this car? if so, where is it? and Can I get all the speakers to work with an after market radio(pioneer) Here is a pic of the radio and speaker cover https://imageshack.us/scaled/medium/248/img0213dr.jpg https://imageshack.us/scaled/medium/705/img0214vj.jpg Testing is the key

You have the Mach audio system. You NEED the harnessbwith the smaller plug. That sendsbthensignal to the amp for the door and rear woofers. Also you will NEED to have a deck with high pass filters or you will need bass blockers for your tweeters. Honestly when indo these for customers I won't even do itnthat way. I typically doma deck and small amp or power pack and crossovers. It sounds awesome when done but does cost more. Id rather take a cut in profit and have a customer with awesome sound as opposed to content with mediocre sound.Gary Sather