I had this problem with an Xj40 when the wiring broke as the window was down.
You have to take the card off the door and find the switch wiring loom.
seperate them and ascertain which one is the live and positive, on some cars the circuits are reversible for up and down but you need to supply a 12 volt feed to the motor in the direction that puts it up.
Many motor problems are caused by dirty wiring contacts mainly through dirt seeping up the door drain channels
Ian Watson, September 2005