My son's 1998 Plymouth Voyager (same as Dodge Caravan) -- with over 100K mostly-highway miles -- makes a screeching slipping belt noise whenever the A/C or defroster are activated. It's definitely engine related, as the noise changes with engine speed regardless of road speed, and the serpentine belt dies much faster than it should. The belt has been changed a couple of times already and dealer mechanics say they can see no reason for the problem.
Any ideas on what is causing the problem and how to fix it? (My only thought is that the belt tensioner has lost its tensioning ability...) Thanks.