In my opinion, the pencil plugs are so superior, that if I started working on a car with series plugs, I'd simply replace them.
I don't think the resistance test will work on series plugs in the same way it works on parallel. Checking the continuity between the first plug and the last should though.
It could be your relay. I don't know for sure if the relay will click if a plug is bad, but I doubt it, because a bad plug typically breaks the circuit so the relay would not be seeing a complete circuit. I don't recall hearing the relay when the strip fuse burned out on my 77 300d.

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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