If your car is having trouble starting you may have many other problems but a dirty glow plug relay I'd say would be the least likely of them all. Do a search, you'll have a million pages to read about a million different causes.

Hope it's not low compression, good luck.

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'02 BMW 325i
'85 300D 450k
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'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)