On 126's there is a wire junction box on the inner right fender low and towards the front. I believe this junction box is the same as the box in front of the battery in a 123. You should be able to open the plastic cover and jump 2 terminals in that box to activate the starter. In a 123 it is terminal 1 and 3. Not sure if that is the case on a 126 but you can experiment. If the starter functions ok when jumped at that box, then the problem is either the neutral safety switch or poor connections in the ignition switch circuit causing a voltage drop to the solenoid. This is a very common problem in 123's and I assume it could be similar in a 126. I looked at a 126 a few weeks ago and the owner described this problem, although it occurred mainly when the engine was hot.

__________________
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