Given the fact that the original loop plugs have been replaced and we don't know how the new relay is wired into the glow light, I would not rely upon the light as an indicator of the condition of the plugs. I would ohm them out to get some sense of whether they are working correctly.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13