Underneath what looks to be ground with the black and white wire is two prongs for the white pigtail hanging. Can someone tell me what that part is called for this pigtail to be put back in?? It’s a 91 rx7 non turbo. And can this cause the car to not start as well? I’ve replaced battery, alternator, starter, and I’ve checked for continuity on all charging system wires. Any help will be appreciated

 

not every plug will be connected to something. check the wiring diagrams here:

Foxed.ca - Mazda RX-7 Manuals

 

As stated above, not all connectors are used on all trim levels. Some connectors will not be plugged into anything as optional equipment could be had and the particular car does not have those options. Mazda is good for that.

Now, looking at the connector you have pictured, it appears that it has never been used as its covered in engine bay grime and the pins are corroded. It could be a check connector for a Mazda dealership tester or it could be for optional equipment. I would do as suggested above and trace the wire in the FSM's wiring diagram to be sure.