This is the flimsy, thin thing I'm talking about:

​​​​​​74510-AL500

I've just cut my one out, it was rusted through. I just can't quite see what it actually does. I know it has a couple of bolts each side that connect to the rear 'kidney' brackets but that seemed to be about it.

 

Bump.

also curious to know.

 

I believe this piece sits above the rear diff, between the subframe and fuel tank. The ends of the W-brace slot or bolt onto it. If that's the case, then this piece is crucial for chassis bracing. Unfortunately I cannot find any parts diagrams with this piece shown. The P/N listed doesn't give any hits for the 350Z. Maybe it's purpose is simply to add mounting points for the underbody plastic aero panels that partially cover the W-Brace just in front of the rear wheels.

-Icer