What you are looking for would involve removing the radio, buying a schematic for it, then soldering a connector at the "appropriate point" in the Audio Frequency section of the radio's circuit board. If you are extreemely lucky, know lots about electronics and can actually find a copy of the radio's circuit board schematic - the mod might work...
... or you can do what beercheck suggested and buy a new radio that has those features. I had a CD/radio in my '93 Sidekick that also had an audio input port. It worked great - and I could play MP3 CDs with hours and hours of music on them. I used the MP3/audio input port to turn the radio into a PA system.
All the above without once heating up the 'ol soldering iron!
I hope that this helps!