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.
- 06 G35 Starter Terminal.jpg (51.57 KiB) Viewed 403 times
You can see the scheme for the battery card on PG-4 here:
https://www.nicoclub.com/service-manual ... 6%2Fpg.pdf