How do I fix the blower direction mechanism on my Mondeo Mk1?

The hot air blower in my MK1 Ford Mondeo (1995) only blows air out of the vents into the car for a few seconds after moving the switch from the off position. If you again move the switch to the center off position and then to a new direction, the air again only comes out of the vents for a few seconds before 'resetting' to blow the air inside the fascia. The temperature selector works fine. What part do I need to change to fix this problem. Thanks.