Scooterotica

Evening all
I want to find a lighting fault on my lambretta
I've read that you can connect a batter charger to the regulator rathing tan having the engine running
It's 12 v ac
I have a Ducati 3 pin regulator
When I remove the yellow wire it's a spade connector.
From this point I need advice

Connect the positive from the charger to that and then the negative to earth on the bike.

Connect negative black of battery charger to a good earth. Connect positive red through a 10 amp fuse to the wire that you removed from the main loom to the output of the regulator. There is your feed to the lighting circuit.

Thank you