I just posted something related to the manual and books4cars.com on the diesel forum, MBDoc. I assume that's the real deal, but it doesn't list the E300 specifically.

If you get a warning when you hit the brake pedal, the switch works.

As for OBD testers, I too thought that I'd really need one for my car, but I probably won't. The computer in that car does a whole lot less for the engine than in the gassers.