So I've got this 2006 Infiniti G35 Sedan AWD and want to put a heavier sound system in. I already got a 370 amp Alternator which i haven't put in yet. I know where all the Grounds go but I can't figure out where or how to run the Power wire. My Vehicle's Battery is connected to a harness-looking thing which i don't think i have to mess with. I will attach a Photo for better clarity. Any help will be much appreciated.

The best place is to gang onto the starter cable terminal pointed by the arrow. The starter is un-fused so you can route from there to your circuit breaker for the amp. The alternator will be a problem, the stock one is fused 120A so you'll need to disconnect the existing wiring from the battery card and add a fused harness dedicated to the alternator. The guys who sell upgrade alternators usually also sell upgrade cable kits, if you didn't get one then you'll definitely need to.

You can see the scheme for the battery card on PG-4 here:
https://www.nicoclub.com/service-manual ... 6%2Fpg.pdf

Ok so I already have all the 0/1 gauge wires. Now all I have to do is run the power wire from the arrow down to the alternator and in between I put a fuse?

Yes, the alternator and amp should both connect there. They should both have appropriate fusible links or breakers.

Ok cool thanks for your help. Much appreciated