Try jumping terminals 1 and 3 in the plastic junction box on the right front inner fender in front of the battery. If jumping those two terminals engages the starter then there's a problem in the wiring somewhere and it could be either your ignition switch or the neutral safety switch. If jumping those two terminals does not engage the starter it is likely either a bad starter or bad connections at the starter.
It's also possible your battery is too low to engage the starter but there is usually a clicking sound when that occurs.

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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