Mate of mine own peugeot 406 and call me with problem. The passenger window start going down, but not up. My first thought was for wear mechanical part and difficult move in the box where the cable roller is, or the sliding plastic element move hard. The voltage measure straight on the motor plug with multimeter show normal and reverse polarity - present. But despite that, with motor plugged in - only down. Disassembling, new grease, and giving power via separate cable - smooth move. Plugged on spot - only.. down. The problem was actually burned/bad contacts in driver`s door button, and under load was not enough current. From there comes the ground for the passenger door motor. How to fix:
1 - pull the button out of the door
2 - pull the down part of the button(use two thin screw drivers)
3 - now you see in the middle one LED and 2 grey plastic buttons
4 - gently (to not loose the touch blue or white pin) with same thin screw driver remove the grey cover
5 - pull out the "spring" contact(seen separated on the images below)
6 - use small nail file to clean the contacts
7 - rebuild , but when put the plastic pin back in the cover pay attention for the dual slot where the "wings" goes.
And some shots
Same work in Xantia as well(same buttons) Good luck
