Renault megan, 2006, electric window fault?

If I try to open the passengers window it goes down, but it also goes down even if I press up. It makes no difference whether I use the switch on the drivers side or the passengers side. The window always goes down. When I stop the engine, an lock the car, the window goes up.
This means the motor is OK, and the switches are working. I have tried holdig the switches for ten seconds, up or down, makes no difference. Any ideas ?

Mike