Vacuum is necessary to direct the air in the cab; otherwise it switches to defrost whenever you accelerate. Dealer wanted $1200 to fix. Problem is a cheap piece of rubber hose that rots out - inside the body! They could have used PVC or Tygon hose, which just gets hard with age, but that would cost 2 cents instead of 1 cent! The result is the battery tray and part of the A.C. system has to come out before it can be replaced. Some people have cut an access hole in the passenger fender well. But after 10 years someone has posted this hack on Youtube, and it worked great for me. It even looks factory.