The Rostra unit referenced in this thread has several methods of pick up. The simplest is with electronic speedo input but magnets attached to opposite sides of the driveshaft is pretty easy as well and works without input from your speedometer.

__________________
Rick
Avid Benz watcher

Previously owned 126's: 2 1981 300SD's, 1987 300SDL, 1991 350SDL,
Previously owned 123's: 1978 300D, 1983 240D, 1985 300D
Previously owned 124: 1994 320TE

Currently:
2004 Toyota Sequoia SR5 206k
2010 Ford Edge 122k
Always looking for the next MB diesel!