I am after the plastic bit that goes between the rear lights,it has 300zx on it with 4 reverse light spaces around it..No idea what its called ..rear reverse lights??..mine has cracked and looks awful.

Commonly referred to as the "centre panel". For reference, there are different types of these available. Options are a standard early model (the most common but getting rarer to find in good condition), the factory optional extra Altia light up panel (the 300ZX text illuminates green - these are quite rare), or the late MY99 spec panel which looks slightly more modern and can still be purchase new (generally, this would match the later MY rear light clusters of you have them).