It depended on which radio/stereo they installed, but my 87 was the same. I plugged a wire from the tape input to the upper inside of the glovebox, where I put a standard 3-pin receptor. I used it for a portable CD player (pre-MP# days, but smae system) until I switched out to Mitsu stereo for a new in-dash MP3-playing CD player.
I found that the Mitsu radio would play the outside signal, but you needed to put a switch on 2 of the din-plug's wires to change inputs.

Don `87 Mitsu 2dr, Rubicon survivor, GModified.