I will add a little ,, I just bought my first Mercedes to tinker with 1979 300SD. Just got thru with a valve adjust today.. I went thru the glow plugs last weekend. Here is what I found..

I pulled the plug off the relay and checked the resistance to each plug , .7 ohms was the average. Use the ohm meter to find which pin goes to which plug..
Then checked the voltage at each plug while the relay was heating.. 11.2 volts to each plug.
Next step was to use a regulated Hewlett Packard 50 amp power supply ,set at 11 volts ,and test each plug at the relay plug harness.
Each plug pulled about 24 amps to start with and then dropped to 14 amps after about 5 seconds.

Since I was doing a compression check thru the glowplug holes, I was going to change the plugs anyway.
With the old plugs out I hooked them to the HP supply to observe the effects compared to the new ones.
The new ones instantly glowed red on their tips. The old ones, pulling the same amperage as the new ones, only smoked the carbon on the tip but never glowed red like the new ones.. Both the old and new were Bosch,, the new ones had a different tip,, the last quarter inch was smaller.
With the new plugs installed the startup was much quicker and smoother,, I could tell a huge difference.