Stuart--the contacts inside the switch burn. This is a very common problem with Volvo's switches, from the 240's onward. You can lower a switch from the command console, but not raise it. But the switch on the door will. These rocker arm switches have a little flat spring contact inside which are pressed down by a tiny ball bearing as you rock the switch either way. Take the switch out and apart and you will see the little ball bearing resting on the flat spring piece of metal, and that the ends of the metal are burnt, as well as the inside contact. Take your trusty jack knife and scrape the points. If one window has failed, then do ALL the switches, as it's only a matter of time before the others start to quit. I've done dozens of switches on my 240's but not yet on my 850 yet. On the old 240's you could disconnect the switch, but someone on a previous post said the switches on the 850 were hardwired. Haven't checked yet, but even if they are, you can repair the switch without having to unsolder the leads. Just don't lose the little ball bearing.

You don't want to replace your switches with ones from a wrecking yard because of the above. They're too expensive to buy new, so learn how to fix them. Dick

P.S. See my reply to the post about ten or fifteen down from here to someone with a similar window down problem.

You have already diagnosed the problem. The contact inside the switch will corrode over time and needs to be cleaned. Pull out the switch and take it to a workbench if it unplugs.
Pop out the black plastic toggle. Be careful, there is a small spring holding down a ball bearing against the contact strip. Clean off the contact strip and put it back. Put a little grease on the plastic bearing holder to hold the bearing in place, and re-insert the plastic toggle.
Repeat for the other side of the car, it is going bad too.

Klaus
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98 V70Rawd(108Kmi), 95 854T(88K mi), 75 164E(173K mi)