I have seen adapters in the $50-60 range that will allow this, but there is a catch. You must have some type of "AUX" device attached such as the CD changer for this device to work. It basically uses the serial data from the auxiliary device to allow the radio to switch to the AUX mode then allows you to toggle between the audio output from that device or from another device of your choice (i.e. MP3 player). I can probably find the link again if you're interested. It doesn't sound like your vehicle is equipped with a factory remote cassette player or CD changer, so you wouldn't be able to use this adapter. I'm not aware of an adapter that would accomplish this without having the tape deck or cd changer installed.
Roger