I posted this awhile back but I don't think I explained it well.
On my 2003 S Type, every time the AC is on, the defroster vents put cold air on the windshield. No matter what setting I put the vents on, floor, straight out, or others, the defroster vents are blowing when the defroster is not selected. This causes the windshield to get condensation on the outside and the wipers are needed constantly in intermittent mode.
Any idea what causes this, or the fix? Thanks.

 

If it does that no matter where you've set the vents it sounds like you've something faulty. There are lots of sensors and actuators all of which a dealer tool can read/control but which ordinary OBD tools can't. So, you either get someone with the right tool who's compatent with it or start doing painful test/replace yourself. Others have posted about the parts etc. so if you want to read what they did get busy with Search