If jumper wiring the terminal block did not work I would think it is the wiring to the Starter Solenoid or the Starter Solenoid itself.

If it can be done safely try using 1 Jumper Cable clamped to the Electrical connection Tab/bolt that connects the Motor to the Solenoid and tap the other end of the Jumper Cable on the posative Battery Terminal.
That that should spin the Starter motor but not engage with the Flywheel. If it does not turn you have a problem with the Motor Part of the Starter.
Jumper wire the Starter Solenoid in a similar manner to see if the Solenoid activates and causes the Starter to crank.

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84 300D, 82 Volvo 244Gl Diesel