If your not getting power out of the relay it doesn't matter what your glow plugs are doing. They die one at a time, yet its possible you had several weak ones.

If you are not getting power out of the relay, is there a way to reset it? If not I think you found the problem.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200