Gregg
I believe your radio is too old to have the "CODE" security feature. What they typically have in this era (although I didn't think the 89's had it) was what is referred to as "dashed line mode". It will have lines across the face that looks like =====.
It doesn't have anything to do with "how" the radio is removed, like what you mentioned "the wrong sequence". It has to do with how long the radio is without battery power. There is a small power source inside the radio chassis, and if it starts to go bad, it will cause the "dashed line mode" to occur even if the car's battery goes dead (ie car headlights left on overnight) for a short period, or if the radio is removed for a few hours. With a real healthy power source in the radio chassis, I believe MB quotes the radio can be without the main power source (car battery connected) for up to 24 hours.
OK, so lets say the power supply in the chassis is a little weak, or even very weak, or the car was stored with the battery removed over the winter, and you have dashed line mode.
Yes, there is a tool that every dealer is supposed to have to reset this type of radio. Exactly how it resets it I'm not sure. How it is used is that on this type of radio, the face of the radio is fairly easily snapped off and on. There is a row of electrical contacts that interface the face plate to the radio chassis. The faceplate is removed and the tool plugs intot he female contacts on the radio chassis. The tool has 2 buttons on it, a red one and a green one. The tool comes with instructions that tells you what to do, it depends on the year of the vehicle as far as what button to push, when to push it and how long to push it.
It seems like MB is protective about this tool, the "core" value of the tool I believe is around $500 if it is lost, and I don't know if they are required to make the tool available to the general public.
My suspicion is that one of the local dealers (former) techs may have kept it locked in his tool box, and that's where it still is .
If you are ever in Madison, WI, my former employer, Zimbrick, I know has the tool and will usually reset the dashed line mode for you for free.

Gilly