Alternator problem - weird behavior

Hi, I recently bought a 1999 GU Patrol. Originally an RD28, it had an engine transplant to the TD42 naturally aspirated.
The problem I have is I have to press down on the connector behind the alternator whenever the vehicle is started for it to charge.
I have tried to clean the contacts, press the connector tighter but still no difference. Once I press on it, the alternator charges normally until next start up.
Any ideas why and a solution please?

Possibly annealed cable at the lug? Try pulling cable out, cut and re-crimp then connect.

or Dry / Cracked Solder Joint in the alternator plug

It is all too easy to turn the connection bolt when removing/fitting the cable and that can/will cause issues inside the alternator.
However it sounds like the issue is in the external connection (as Hodge and happy gu said)