There is a black ground wire coming out of the harness where the fusible link come out and attaches to the starter. This ground wire connects to the fender well across from the starter.

My question is what is this ground wire grounding?

Thanks in advance for the help

I am talking about a 1971 240z.

I don't have my book but off the top of my head I would say chassis ground.There is also a frame ground.With all the rubber and moving parts between the two a few common connections are needed to keep the same electrical ground potential among all parts.Remember your lights that are on that fender need a good ground to burn.