I don't know about the '79, but on my car the glow plug fuse and relay are housed in an approx. 1.5" x 4" black plastic box mounted on the drivers side wheel well sort of behind the lights. Pull the cover off and see if the fuse is not blown. I think the '79 also has the series type glow plug system unlike the later models which have a parallel set up. So, the problem could be with the wiring. I think the Haynes manual provides a good procedure for checking the system.
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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold
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