What does this plug go to?

[Asakura]

I just did the swap and this is the only problem I've really come across so far. This could be pretty obvious but I'm a little stumped on what it goes to. It's somewhere between the starter motor and the alternator, and has a rubber boot around it I'm guessing as an insulator. Any help is greatly appreciated.

Here it is:

Thanks!!

[Seishuku]

Oil pressure switch?

[jr_ss]

Goes to the factory oil pressure sender...