voltage will go to the plug as long as the fuse has not failed on the GPrelay... to test the plugs, you have to measure the resistance to ground. from each plug in the 6 pin connector to ground. the ones that measure less or more than 1 ohm are bad. more than two bad and the light on dash quits.
http://www.dieselgiant.com/glowplugrepair.htm

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!