how do I remove the 3rd break light via the trunk? how do you access the second screw?

If I remember correctly, you can only almost get to it from the trunk. Service manual:
http://www.nicoclub.com/manuals/G35/...5_Sedan/lt.pdf
(p137)
says to remove the rear parcel shelf:
http://www.nicoclub.com/manuals/G35/...5_Sedan/ei.pdf
(p41)
But that means you have to start by removing the rear seat, seat belt anchors, etc...

Then finally you get to remove the two screws holding the center brake light...

Drill a hole in the sheet-metal so you can get a screwdriver on it.
It's a very tight squeeze figuring out how to wiggle the bulb out and back in but it will fit.
Just pay attention to the wire routing and don't put the new one in backwards/upside down.
Where did get it and what new bulb are you putting in?
I've had this one for 2yrs with no problems.
https://www.ebay.com/itm/22267516653...m=222675166535

I just moved mine back and forth until the tab broke off, pulled out the old light, put in the new, tightened down the one screw. Stays in place, works fine. Took me 5 minutes total.