I am having a similar problem but since I can always get the car started by jumping the wheel well terminial block I have not fixed it yet because I'm a procrastinator.

Since your NSS is new, it is not likely bad out of the box. It could be NSS alignment, high resistance in the ign elec switch (power comes in at pin 30, comes out at pin 15, high resistance/ bad connections in the circuit path from the battery to the starter. The path is:

Battery, starter motor big terminal, light switch pin 30, ign switch pin 30, ign switch pin 15, NSS pin 3, NSS pin 4, terminal block violet wire, starter motor solenoid (small terminal).

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked