This sounds like the solenoid wire. Mine checks out visually and with continuity test. I was able to start my car by jumping battery + to the screw terminal on the firewall that hold the small wire leading to the starter.

Since the transmission was the last thing worked on, I will focus on that. NSS and ground braid some to mind.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 190k miles
97 C280 150k miles