The new replacement radiator I bought for my 99' Tahoe has that same port with a rubber cap. Came that way outta the box. My Tahoe has rear air but there was no OE hose going to that spot on the stock radiator. I was wondering what the heck it went to.
I think it’s used if there is coolant plumbing going to rear heater core and blower. From what I’ve gathered in a little bit of reading some have it, some don’t.