Access to Cassette interface on GMC Sierra

My 2001 Sierra had an option for a "remote cassette" deck that is meant to mount in a tray under the dash. If so mounted, the "AUX" button on the radio turns it on. I would like to add an external jack (for an MP3 player) that would play through the AUX signal input. Does anyone know where the connections are for this and how to get the radio to recognize the AUX input? Also, when I press the AUX button nothing happens, which makes me believe there is a jumper or something that tells the radio that a cassette player is installed.