On a 124, there is a three-wire block under the windshield cowling on the driver's side. From foggy memory, the middle wire goes directly to the starter solenoid, so applying 12v to that should engage the solenoid and make the starter spin. When I had a flaky solenoid, I could usually coax it to start with that method. The voltage loss through the ignition switch and NSS was just a little too much to provide enough umph to kick the solenoid.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC