Changing the speedo cable isn't too tricky - for the w201 it was as follows:

1. disconnect speedo at cluster and under the drivers carpet
2. disconnect speeod at tranny (remove the retaining screw all the way)
3. pull the speedo cable out of the car (in the w201 you have to go through the firewall, on the 124 it is under the drivers carpeting)

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE