I'll second what Niel has said. Rather than try and diagnose the complex wiring harness, I have had guys simply replace the whole harness to get rid of gremlins.
The diagnostics are through a special Mercedes diagnostic scanner known as the star diagnostic tool. It is one thing to buy the scanner from ebay or whatever, but then you need the software, which expires frequently. Here is a good article on the subject:
http://www.mercedesmedic.com/mercede...-need-to-know/
As cool as it would be to have a surplus DND G-Wagon, at the end of the day they are just a unsustainable SUV. Give me an open top hummer or Willys Jeep any day.