Does anybody know where these plug into?

If you're talking about the starter relay nothing plugs into, all the wiring there is screwed under terminals. It might be part of the stupid seat belt interlock wiring.
I don't believe 75 had that, only 74

OP 75 is a year that no one has a free service manual download available. If you do not have a service manual, you really need to pony up and get one either digitized or reprint paper