Originally Posted by mercmad6.3

The speed sensor is in the trans itself,amongst the clutch packs . there are two magnets which collect the signal from a toothed wheel passing over them. The magnets are in the plastic contact plate,so if you ever have to change a contact plate you can see the them. On one of mine, a needle race has come apart in the trans and the speed sensor magnets are covered in needle rollers. Yes it drove until the number of needles prevented a signal reaching the magnets and the drive was lost.
i cleaned the needles off,replaced the plate and lower pan and proceeded to use the car for a few more weeks until the trans gave up completely.
So.. If you want to be able to get a speedo to read the signal from the trans, the loom coming out the side is where you start. the Mercedes WSM has all of that info (nothing much on line regarding which color wire but suspect it will be typical DIN and white. You only need to come up with a device which will power a digital read out when activated by the ignition key .