@MJR
Are you sure that this wire harness you found, really was a battery tender harness? I remember a post years ago from Stig here about his Like scooter when he asked the mechanic about the diagnostic harness in the battery compartement. And the wrong answer from the mechanic was: "Thats for the battery tender".... But I have now edited my post above, because I don´t know about Kymcos largest models wheather they have it or not.
Normally all new scooters come with empty, dry, removed batteries with an extra acid pack in the underseat bucket on the crate. So normally all batteries need to be filled with acid first, then charged and finally installed into the battery compartement. At least thats the procedure for smaller to mid size scooters. Most likely also for large scooters, because it´s a safety issue for transportation/shipping.
So why should a manufacturer then factory install a tender harness with a certain plug that then most likely will not fit to a battery tender you have to buy afterwards or you already have at home which has a most likely different plug system?
@DakLak
The threadstarter already mentioned, that he has bought a NEW battery. So the battery should be fine now.
So, really, I would do the most simple things first:
Check the glass fuses.
Then do some measurements with a voltmeter.
Then check other parts for electrical/electronical damage.