When measuring glow plugs use either engine ground or battery ground and measure each of the two outside pins on the valve cover connector. Each outside pin is a glow plug. Also set the meter to the lowest resistance setting available. Each should measure ~ 0.5 to 1.2 ohms.

If a reading is a lot less or a lot more then removal of the valve cover is necessary for further inspection. Once removed you can put the meter on each glow plug directly eliminating the wiring harness.

 

If you did the SD VCG's the same problem would be present as well. All one piece.