Alternator problems 94 ls 400

[Yabwa]

What could be causing my alternator(it’s brand new) to not charge my battery and the serpentine belt to squeak.  also what is this wire in the trunk for. 

[Yabwa]

Also whenever I jump the car and leave it running after about 15 to 20 mins it starts jumping for 150 rpm to 200 rpm repeatedly 

[Lexus Owners Club]

I wouldn't say that wire relates to the alternator charging but it is definitely worth crimping it back in and see what happens.
if the belt is squealing then this would more than likely be the reason for low alternator output as its not efficiently driving the pulley.
jumping idle speed is not related to the alternator I wouldn't think.

So, tighten or replace the drivebelt and reconnect the loose wiring.