From what I have been learning so far, a speedometer should be replaced with one from another car that has the same engine. Also, older cars have the speedometer connected by a cable and newer cars have electronic versions. Another difference would be an anlolog odometer as opposed to a digital one obviously. I think 1993 was the last year for the analog.

For example in my case, I have a 1989 300ce with the M103 engine so a 1990 300ce with the M104 engine would have a speedometer that is not compatible with my car. That's probably what happeded to you in the case of the 500SL.

You probably can use one from the following cars:

1990-1993 300SL
1992-1993 300SE

Don't quote me though. I am only offering you what I "think" might work.
Good Luck. Hope this helps.

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz