I'd buy two of them. There was a thread some years ago where someone installed one in the oil pressure gauge line. I think the problem is finding the right metric fittings to T into the original fitting.
There will have to be two designs, one for the 126 electric sender and one for the mechanical gauge.

It might even be possible to use the seatbelt buzzer. Just disconnect it from the seatbelts and wire it into the pressure sensor.

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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