For the record, I'm not sure if this is true for all EA82's, but I have an '89 GL wagon with the radio and separate stock cassette player.
Like many, the only two music players I really want in my car are a radio and my phone/mp3 player, so I enlisted the help of an electrical engineer (not required) to help me install an auxiliary cable in my stock set up. What we found was an extremely simple and elegant method that kept the stock appearance and retains functionality of the cassette player.
-Remove radio and stock cassette player from dash (clip on frame, six screws)
-Disassemble cassette player (clip on top, all visible screws on frame)
-Locate where the black, 8-pin DIN cable that plugs into the radio from the cassette player is soldered to the circuit board
-You should see six wires going into the circuit board (including the meaty ground cable)
-Now this is where my memory gets hazy, and I'm too lazy to take it apart again, perhaps upon request, so hopefully someone can chime in. If I remember correctly, the first two wires closest to the ground on the board were right and left sound, but I also can't remember which was which. This is where you would splice in your left and right sound wires from the auxiliary cable you purchased, and the ground to the ground, obviously. It's easy to test for, however, since the radio automatically shuts off when the circuit is completed.
-Then just find a way to gracefully run your cable into the cabin and enjoy any auxiliary device you wish! All you need to do is plug it in and the radio will shut off to allow the device to play.