My 2003 G35 sedan, Has been acting weird i crank it for the first time it has 14.6 voltage from the DMM. When i get to my destination i start to crank it i got 12.7voltage it has the battery and break light on and when i arrive home and turn off the car and try to start it for the third time it will die off and i check in with Autozone they told me that the battery is good. If anyone can help me with this. I do have a aftermarket headunit and a single 12inch sub paired with a 500watt amp. Please help me out

 

Using a DMM to check a battery only works if you test WHILE CRANKING THE ENGINE.

Sounds like your alternator is going out since the car lit up the battery light while driving, that means voltage is dropping below 12.5v while the engine is running. It should be 14.5v while running.

You can start by disconnecting the head unit and amp though, might have some kind of parasitic draw killing the battery. Run for a couple of days with it unplugged and see if the problem persists.