If your radio is in the upper slot there is also a bar attachment on the back of it. You can get at it by removing the glove box (you will see the radio and the bar once the box is out). Or you can go through the speaker cover in the center of the dash (the bar attaches to the center wiper assembly).
If you have a CR-712 radio, its not great. I still use mine because I like the stock look of it. The new radios are too flashy, and they don't have a knob for the volume. Unfortunately the CR-712 does not sound that great. I have a Graphic equalizer, and power amps from a 740 and it still does not sound very good. Its really heavy on the base, and low on vocals. If I switch in an Alpine head unit it sounds way better.
You might also want to check the wires for breakage in the hinge of the doors, and I have found that the windows can catch the speaker wires when they are rolled down, and pull them off the speakers.

To the left of the radio there is a square plastic blank, pop it out. You should see 2 screws that hold the radio bracket to the dash. Remove the screws and pull out the unit.

Man I'll tell ya' ... the stock radios just suck. You're better off replacing the radio and speakers and throwing away the amp. I believe the amp in the '89 powers the rear speakers and is located either under the steering column or glove box ... I can't remember. I have all of the instructions from Crutchfield if you want me to fax them to you. Just e-mail me at classicbrick438@yahoo.com and give me your fax number.

Regards,
--
Bill Crump - ('91 245SE & '93 244 "Classic")