2002 tahoe radio harness

hey guys we got our new tahoe today and i'm trying to put my dual 7" dvd/cd player in and i got my installation kit and already had a plug. i got everything hooked up and have no sound. it has the factory cd player with the tape player down on the console. the back of the stock radio has on big plug on the back and one small thin plug on the other under the antenna plug. when i hook the stock cd player back up with both plugs it has sound, when i unplug that thin plug i have no sound. all the aftermarket harness' i've found are the same one that i have already, none of them have an extra plug to plug in. has anyone had this trouble before or delt with this? i'm going to call best buy tomorrow, i know i know but its the only place open on sunday in my area. all the actuall shops are closed til tuesday. please guys i need some help, i wanna get my truck put back together so i can drive it this weekend......also it has OnStar and the non bose system


thanks


andy