i had that problem too and i just gave up and have it set up in stereo because honestly ALL OF MY MP3 is STEREO! so who cares?! just so u can tell people that u got 4.1 in your car? sorry i cant help but untill u find the plug in just listen to it in stereo.
my 5 ch amp had a switch that i could flip to choose 2ch/4ch/5ch i have it set on 2 and the cross overs and gain controls are what i use to fade front to rear for now.