You need to trace the circuit back to where you see the 12V again. What's happening is that you have a high resistance somewhere in the line. When there's no current flowing (glow plugs disconnected) then you'll see voltage wherever there's any kind of connection - V=I*R: when current is zero there is no voltage drop across any resistance, so you see the same voltage everywhere in the circuit. The second you start drawing current (glow plugs attached) the most voltage is dropped across the largest resistance. Glow plugs present less than 1 ohm so anything larger than 20 ohms is going to make it look like there's practically nothing at the GPs.

First thing to check is the GP relay fuse. Take it out and make sure its not in two pieces. The GP relay is connected to the terminal strip on RH fender well, same place as the alternator. These voltages are always hot so be careful. Clean all these connections. Make sure the battery has a good connection (does the starter crank okay?). Clean the connection on the glow plug relay where the big wire attaches. When you put it all back together you should have voltage at the glow plugs. If not look for where the voltage goes from 12V to nothing under load, like a broken wire or a bad terminal connector.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)