If you still have the old filament/loop type glow plugs if one plug burns out none of them work.
It requires you to change the glow plug wiring but the updated pencil type adapter glow plugs work better and when one plug dies the rest still work.
Somewhere in the repair links is threads that offer some instructions.
DIY Links by Parts Category - PeachParts Mercedes-Benz Forum
Sometimes the stock relay does not work with the adapter glow plugs and you also need to change the fuse to a higher amperage.
People generally make their own wire set. One of the main things you have to remember is that you will discard the ground wire that is at the very end front of the older glow plug set up. It may go direct to the cylinder head or go to a temperature sensor.
This could be one of the links to the conversion however the kits seem to be long gone:
http://www.peachparts.com/shopforum/mercedes-parts-reference-library/178765-f1101-88322-kit-converting-loop-series-gp-hotter-pencil-parallel-system.html
In the case of the adapter glow plugs Bosch or Monark plugs work. People have also used another company that I Connot spell right Heuco or something like that. If they are not the adapter type glow plugs don't use Monarks for later engines with the smaller glow plugs as they do not last.