So I was under the car today and noticed this. Some sort of extension plug. It routes by the spare tire well and seems to go in the direction of the battery. Any ideas? I don’t want to plug it in until I know exactly what it is.

 

Looks to ME like a charging plug, so far as I can tell from what you posted. You'll have to take off the battery cover and such to see what it does up there. It may be just leads TO the battery, but if you're REAL lucky there may be a previously installed CTEK or similar already installed.
Chances are though that it's just a home-grown quick-disconnect for charger leads.