1/4" vacuum line with T coming out of inner fender on Pass side???

I have a 1/4 inch vacuum line coming out of the inner fender on the passenger side into the engine bay of my 03 Tacoma 3.4 V6. Goes into a Tee and continues to a spot in the front under the air cleaner box, where it goes back out through the inner fender behind the headlight. At the tee, where it comes through the inner fender, one side of the tee is open. Any Ideas what it is/does, and can I just plug the open side? Not my truck.... rebuilt the engine for my son-in-law, who drove it for 167,000 miles without changing the oil. I'll start a new thread and post pics for that mess.