That's the plug. In the back of your CD/Business radio. You need to buy a metra harness from ebay for it to be clean. It's about $12 with shipping. In your case you could unplug the cd/business radio and move it to your trunk. (that's where the dsp amp is) From there plug the aftermarket metra harness into your stock cd/business radio and make the appropriate connections. (listed below) After that your eonon unit should plug and play with your factory radio harness that is up front in your dash still. Hope that helps.
Connect pin 16 from the round pin plug - - with the wire from pin 4 on the Big AMP Plug
Connect pin 9 from the round pin plug - - with the wire from pin 8 on the Big Amp Plug
Connect pin 15 from the round pin plug - - with the wire from pin 15 on the Big Amp Plug
Connect pin 5 from the round pin plug - - with pin hole 5 on the DB25 plug
Connect pin 7 from the round pin plug - - with the IBUS wire on the smaller plug on the amp (should be white with yellow spots)