Ahh salvage title cars are my favorite, although I don't know if I would have the patience and pocketbook for that kind of gamble today though. However my last rebuild was a Bonneville and nothing at all like a LAN equipped G.
The window control system on this car is very simple but also complex. The power locks system is also part of the LAN network. You are going to need to learn how this system works before we can even really discuss what's wrong.
http://www.nicoclub.com/FSM/g37/pwc.pdf
Read up.
If you don't have a good multimeter you will need to get one. This is not a first project type car so I really hope you have some experience.