It is unlikely that the motor is toast. It is very likely however that the terminals in the switch have some corrosion. This very thing has happended to both volvos I own. All you have to do is remove the drivers side switch clean the terminals with a wire brush reinstalll it then do the same to the affected door. I think there is a more detailed FAQ. becarefull not the lose the small metal balls in the switch as the window will do weird things with out it (ask me how I know). Does anyone know where to buy those balls anyway?