How important is D- ?

I was just unplugging all the wires in the bay for the big engine drop and spotted something I never noticed before.
My regulator only has 2 terminals. The D- (brown) wire on the connector isn't connected to anything.

The van ran fine for years like this. Just wondering, would it be better to have D- connected to something?

i think its earth, cant quite remember... if its not there its most likely earthed elsewhere. any1 clarify?

I think it's earth too. I wasn't sure though. I was a little worried because the wiring diagram doesn't show it as being earthed.