If you have a push-pull knob,you don`t need the relay,just keep things simple...
Are you sure you`ve got any relay at all??
if you insist on having it,check the manufacturers` catalogues for original relay part numbers:
http://www.beru.com/english/produkte/produktfinder.php
http://www.bosch-automotive.com/ecat-online/
www.autoteile-shopping.de
Get rid of the series glow-plugs and get an upgrade kit from Bosch:
0 250 201 955
Kit Includes: 4 Glow plugs, 0 250 201 044; 1 Control Unit - 0 281 003 005, 1 Service Kit = 1 235 345 001 - Nut; 0 332 002 156 - Relay; 0 354 010 001 - Fuse Holder; 1 904 520 017 - Fuse;
...or just buy 4 glow plugs Bosch 0 250 201 044 or Beru GV 626 (0100221311),or buy the simple non-relay kit SR 002 from Beru(page 41):
http://www.adpdist.com/beru_glowplugs_application_na2004.pdf

...make jumper wires(min.6mm.sq.) ,replacing the resisror wires between the plugs and get rid of the grounding cable from plug 1 to the cyl.head:
http://www.dieselgiant.com/mercedesdieselloopglowplugrepair.htm
http://www.dieselgiant.com/glowplugrepair.htm
The car will start much easier...