You can bypass the ignition switch and do what your mechanic did by jumping the 1st and third wires (screws) in the junction box on the right front inner fender. This will allow you to get by until you resolve the switch problem. There's a thread here somwhere describing your problem as caused by dirty contacts and a description of how to clean them.

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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