Does a 79 have the loop/filiment type glow plug system or the pencil type Glow Plugs?

You have not mentioned looking for a short circuit like on the wiring or connectors.

If you have the loop/filiment type glow plug it is possible for the short to be inside of the Glow Plug or due to carbon build up.

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84 300D, 82 Volvo 244Gl Diesel