Two most likely sources of your problem are the ignition switch and the neutral safety switch. Have you tried starting the car in neutral instead of park or keeping the key in the start position while moving the shifting up and down thru the gears and into park? Also, sometimes if the ignition switch contacts are dirty, holding the key in the start position for 10 seconds or so will eventually result in the starter engaging.
If it's the neutral safety switch, you'll have to replace it. If it's the bushing in the shifter, you'll have to replace it. If it's bad contacts in the ignition switch, there is a post on the forum somwhere describing how to clean the contacts.

__________________
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