I have a 94 Chevy Silverado with the base cassette deck in it. now I found a 'premium' deck at a junkyard. this one has the equalizer, and more cassette functions on it. however, on the back of the box theres the regular wire harness that plugs into the control module, and theres a two lead plug on the corner. what plugs into this port? I dont remember seeing this plug on the back of my stock one. is this just like an auxiliaryplug for aftermarket stuff, or what?