Summary of the good questions.
Did you ream or otherwise clean the Glow Plug holes.
What brand of plugs were you using.
More questions.
In the couple of days the car started OK did it idle well and was there any smoke.
How many miles are on the Engine.
Do you think swapping the GP Relay and a (+) or (-) effect or caused no change in the failure rate.

Question for anyone. Can a poor ground cause the GPs to fail?

Some Members on this and other forums believe that if the GP holes are not reamed or otherwise cleaned out the heat gets transfered through the carbon to the block instead of heating where it is supposed to. Others think it might insulate the GP and cause it to hold the heat in in the plug itself.

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84 300D, 82 Volvo 244Gl Diesel