do I have to replace the whole tail light assembly if the smaller inner light is not working? Or does the lens come off so I can just change that bulb? The outside (larger) one is working fine

 

Attach a picture? I’m confused.

 

No...it's a simple bulb replacement, might take you 30 minutes! Drop by Infiniti, pick up a bulb, that's because there's numerous bulb sizes involved...especially reverse bulbs.
Oh yea, gsff94 "Welcome to the G35 Driver forums!"
Gary