I'm not sure how the W124 is but it should be similar to the W201. It's on the back of the speedometer next to the speedo cable input on the W201 instrument cluster, instrument cluster in the W201 is a bit of a pain to deal with. May help to detach the speedo cable from the transmission to get enough length.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.