My fusible link has blown and the car is dead. I have a new link but need to identify the cause, anyone have any tips?

 

Which fusible link, the main 120A one? You definitely want to do some testing before you reconnect the replacement. That's like blowing your house's main breaker.

Has the car been in a wreck? Battery polarity correct?

Power & Ground manual (for an '03 sedan) is here:
http://www.nicoclub.com/FSM/G35/Sedan/2003/pg.pdf

Fusible links B & C are 100A and 80A and simply sub-fuses connected to A, the 120A. You need to find what is shorted to ground before you re-energize things.

Good luck.