Vw mk1 1.3 Jetta, alternator not charging and no battery light on dash

1981 mk1 jetta 1.3 GF

Having issues with my alternator not charging the battery.
The battery was new 3 months ago and the car is not on the road yet. Had the car running with the  lights on and they just go dull. Car continues to run but does not start again once turned off. Leave it about an hour and it starts again. I've never seen a battery light on the dash with ignition on or when the engines running.

The alternator has a 3 pin plug one side with a small blue wire and two larger red wires. The other side has one single connection with no wire attached to it, could this be the problem

Any help appreciated.

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volt meter needed mate

If I jump a wire from the back of the blue wire on the alternator to the battery earth terminal,  my battery light will come on so I know the bulb is working at least.

Checked gearbox earth wire all good.

Had a look at the brushes all seems good.



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