My '91 came with a factory phone. My wife and I both have Nokia 6160 phones from work, so it didn't make sense to get service on the old analog phone in the car.
Here's what I did:
1. Remove interior components of stock phone system, including handset and coin-tray insert. If you don't have a factory phone, don't bother with this.
2. Install Nokia car kit "brain" in rear wall behind driver's head.
3. Install Nokia handset in armrest (same place the factory handset was located).
4. Place Nokia phone "cradle" mount in the coin tray, so that the phone lies flat just behind the shifter.
5. Connect speaker outputs from Nokia unit and radio mute to the factory center speaker (between the seats) and the radio mute input. The radio mute input is on the wiring harness at the center speaker, as it is used by the factory phone.
My "rear speaker" between the seats is no longer connected to the stereo system - it is dedicated to the phone. The Orange/White wire in the wiring harness is the radio mute input.
EDR
91 Black/Ivory #3012