How do I replace the vacuum line in a 1999 GMC Safair that controls the air cond. blower I can see the part of it near the front fender. I have rubber hose to replace it but can't find other end.?

I just did this on a 2000 Asto van-look behind the tire on the passenger side--pull back the rubber mud guard attached to the fender well- behind it you will find a metal ball with a nipple protruding out of it. push the other end of the vacuum on to that nipple securely. That should do it. I just wish I could find what the metal ball does. Why does it need vacuum line. Hope this helps someone