The nozzle had been sheared off on the car I bought. I repaired this, and posted a writeup with photos here:
http://www.volvoxc.com/forums/showthread.php?2972
8-2004-XC-missing-tailgate-window-wash-nozzle&highlight=rear+window+washer

First, check the washer fluid level, it needs to be higher than the rear pump.

Make the rear window wet so that you don't grind dust over the glass. Then push the stalk to get the pump to run. If you hear it, with the radio OFF, let it run for a while and check for fluid (15-20 seconds). In your climate, the pump may not have run for over a year. Or if the previous owner is like me, I never use the rear washer, just clean it at a gas station.

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Keeping it running is better than buying new

The fluid should shoot straight out of the little tube in the center of the shaft. Then there should be a sort of L-shaped plastic piece snapped on that reflects the water stream back onto the windscreen. Different model years "park" the wiper to either the left or the right so you have to keep that in mind when searching for the plastic piece - it's NOT interchangeable between LH and RH.

If you're not getting water flow to the sprayer....

Both front and rear washers are supplied from the same reservoir. Make sure it has fluid in it. It has two pumps (Fwd and Rear). If you don't hear the pump running when you activate the spray, check the fuse. The pumps are mounted right to the side of the reservoir. The bummer is that you most likely have to pull the RH side of the front bumper skin loose to get access. If the pump is dead (unlikely?) give me a shout. I've got a couple of them at a special price just for Brickboarders (like you pay postage only). They're going in the trash soon along with the reservoir that Bambi cracked late one Christmas night.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)