Before unhooking anything, I checked for 12 volts at each glow plug while the relay was "on". All 5 plugs had 13 volts. The power supply is OK.

Then I started looking at the individual glow plugs.

I disconnected the wiring harness from the glow plugs (at each glow plug). Then I made a "hot wire" to hook to the positive terminal of the battery. I used this jumper wire to check each plug, looking for some sparking/arcing as the wire touched the trheaded terminal of each glow plug. None of the glow plugs seemed to be drawing any current when tested this way (i.e. no spark or arc when the test wire was applied). Then I pulled the first glow plug out of the engine, grounded the outer housing, and put 12 volts to the end terminal. It did not get hot.

That was my "shade-tree" test method. Let me know if this methodology is not applicable.

SteveM.