I have a 617 W126 300SD and on the drivers front fender well is a box with the wires from the glow plugs, I believe the same is true for your car. My brother had the same car as you different color. If you pull the top off the black box you will see that there is a multi pined connector with the wires going back to the glow plugs. If you pull the plug you can test the viablilty of the glow plugs using a volt ohm meter, either analog or digital to see how many ohms resistence each glow plug drawing. A new glow plug draws approximately 0.7 ohms (Bosch French GP's) new. In circuit you may have a different reading but it will tell you how many GP's are still working and how many are not working. The most definitive way to test a GP is to remove it and with heavy jumper cables carefully connect the Black wire to the silver body of the GP and the Red wire to the threaded end and watch the tip of the GP get red hot within a few seconds. Best of luck with the new car, this site has a wealth of information for you to learn.

__________________
Gary
OBK #37