You'll have to double check the part numbers, but I am pretty sure the 126 uses a different kind of speedo than the 123. (the 126 that I have does anyway, although it's a 88 420SEL) The thing that usually goes wrong with the 126 speedos is this little tiny plastic gear. Some turn crazy (similar to the 123 affliction), mine just fell apart.
If you can take a gander at a 126 speedo sometime, peek behind the dial on the left side. You will see 3 gears...one of them really small. That, is the one that goes. I have thought about finding a gear like that at a hobby shop, but had no way of knowing the dia. nor how many teeth it had...so I shelled out the $ for a rebuild.
Good Luck
-Larry