Since you already have the transmission with the electronic sensor, this swap should be relatively easy, I think. I'm not that familiar with the 123/126 instrument clusters but all you really need is to get a 126 electronic speedo installed in the 123 cluster. The wiring is simple. It's not one wire, it's three wires. The single green/yellow wire in the center (if present) is an output to the cruise control system. The three (or four? I forget) wires are the input to the speedo.

The schematics should be in the ETM (Electrical Troubleshooting Manual) for the W126. That will show you how to wire it up. You will almost certainly need a converter box though, like the one from Yellr.com (for 2-wire inductive sensor!), or from Dakota Digital. This will let you adjust the signal for your specific gear ratio and tire size.

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