Hopefully, Moregolf is correct. If not, it may be the windsheild wiper transmission (windshield wiper control arm which connects the wipers to each other and the wiper motor). It is located under the plastic cowling below the wiper connections. It is a ridiculously complex group of sheet metal pieces with multiple connectors which drive and connect your wipers. Over time the metal suffers fatigue or if you use your wipers to remove heavy snow or inadvertenly turn on your wipers when they are frozen to the windshield, the transmission linkage can break. Mine broke two years ago. I do not recall ever having abused it, although I bought the car used.

The replacement part cost $165.00 two years ago plus two hours of labor to remove and reinstall it. I found one in a junk yard for one third the price and tried to install it myself. Unless you are very good at mechanic, do not try it. You must have a special template to position the wiper transmission linkage in perfect coordination with the wiper motor or it will wipe your front hood or your car roof instead of your windshield. After hours of torture, I had a Volvo tech friend of mine do it. It was worth the money to pay him.

Dan