Ok here's my 2¢: I don't know anything about an hvac or switchover valve (????) nor do I dispute what MBDOC or Cdplayer are saying as they have both personally given me advice and even walked me through repairs over the phone, but I had a similar problem and it was the heater valve located on the right side of the battery on the passenger side. I tested it by removing the valve, which is fairly easy because its only connected by 2 hoses. I lifted up on the flap of the valve to where it should've been closed and I blew through the one end and air was coming out of the other end which was not suppose to happen. I later ran water through both ends with the flap open and supposedly closed and water still flowed right through it. My temporary solution was to cover both ends of the valve with electrical tape and pennies over the ends and hook it back up, this way I atleast stopped the heat from coming out simultaneously as the cold air from the a/c. However, I still have not yet bought a new valve yet, but so far so good. You might wanna check the valve and see if its working or not.