I did two things. I slightly pressed down the channels that the doors track in. This increased the friction to keep them from opening when I turned a corner or went over a bump. Second, there are screws holding the back plate of the heater doors to the body of the car. Gradually back one of these out until a little friction prevents the door from sliding (if the screws are very short, or set on an angle this may not work). Mine now take a nice 'tug' to open and they stay where I put them!