Audio Systems: 1990 Cadillac Delco OEM Aux Input, cadillac sedan deville, preamp section

Question
I've been looking for alternatives to a tape deck adapter for my MP3 player in my 1990 Cadillac Sedan Deville.  The tape deck no longer works.  I came across this stereo (http://cgi.ebay.com/ebaymotors/Cadillac-Radio-Tape-Delco-Symphony-Caddy-90-91-92...) on eBay and was wondering if you would have any idea how this guy modified an OEM stereo to override the radio when something was plugged into the aux port.  Any ideas?  Or any suggestions for adding an aux port in any other way?  I don't want to go the FM transmitter route (I've tried it before and never could get a clear signal.

Answer
open the radio , find the preamp section of the tape player and solder 2 rca jacks there,
you need to know electronics and also need a switch to turn off the tuner and turn on the tape preamp you will do that on the preamp ship of the radio
good luck