The way you have to wire a Bosch loop plug is a little tricky if you don't have the proper info close at hand.
These plugs do not ground through the body, there are two terminal at the top of the plug. There is a ceramic insulator that keeps the two terminal isolated, if you missing those insulators that's the first problem you need to correct.
http://img.eautopartscatalog.com/live/W01331638701BOS.JPG
If your books don't explain how to wire up the plugs I can likely get a diagram from one of my Mercedes engine manuals that will give you enough info to get the job done.
Tracking down a voltage dropping coil might be hard. I know it can be sourced from Mercedes, but I'm sure you could convert the system to parallel plugs for about the same amount of money and you'll get an automatic controller too. Google part # W0133-1606011 to see it.
If you want to go cheap you could just buy 4 plugs, part # 0 250 201 044 and use the old switch and completely eliminate the need for the dropping coil. That's all I've ever done on my engines and it works great.
Jason