http://www.dieselgiant.com/glowplugrepair.htm
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm
Second link is for hoop style GP... not on your car but while your discovering the wonders of GP ...
Have you tested the replay timer?
If the relay tests OK and the GP's test ok then you have the blub and ign. switch left to test.
I have not 'hot wired' (actualy a jumper) the GPs but I'm sure someone on this site has so you can verify everything works beside the ign. switch.
You may have a bad ign. switch.
There is a past post with details of bypassing the ign. switch.