Radio replacement with CD changer
 

I have 97 Montero Sport with AM/FM radio, cassette deck and CD changer. I'm planning to replace it with Radio/USB/SD player. At the back, it has 2 round plug, 2 antenna plug (big and small) and 14 pin plug with 5 wires. I think the 13 pin round plug is for CD changer, the 6 pin round plug maybe for speakers, not sure. Anybody have done this before? assistance greatly appreciated.