Inside the steering wheel, behind the airbag (be very careful around this when removing)... there are 4 plastic parts that hold contacts. If yours is like many of ours, the post have probably broken. More often, it seems, is that the horn will get stuck on, but mine didn't work and then worked constantly. It's a rather expensive fix to replace because Volvo wants to sell you a new steering wheel. Search the logs here and on Volvospeed as there have been some that have had success fixing the broken plastic... Mine were too far gone to fix and I had to retrofit the whole wiring harness.

Good Luck,

Shawn