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What’s wrong with my alternator?

  Megane RS
At track on monday night my (172 cup) battery light and stop light were occasionally flickering. After a few sessions, the car wouldn’t start - seemed like an obvious flat battery. Got a jump from a booster pack and managed to drive home - 30 mins long. Seemed to me like the battery was knackered but it ran fine on the alternator?

So, ordered a new battery, came today and just fitted it. Started up fine, but according to multimeter it’s not charging - so i’m thinking the alternator is indeed kaput (weird how it got me home for 30 mins after a jump though?).

one question, what is the little single wire plug on the alternator for? It makes no difference when i start the car with it plugged in or unplugged - still no charging but also no battery light showing whether plugged or not.

if anyone has a suggestion on how to test the alternator/little plug for problems, would be greatly appreciated.