Your local volvo dealer or volvo parts should carry the switch. It is identical to the 1998 S90's antenna switch. If you order the S70 switch it may be a different color if you're installing it in an 850.

I installed on last year. Look for the radio wiring diagram in Volvospeed's Bay13. You'll have to tap the antenna remote wire if you're not buying the wiring harness.

The switch was $25, no need to remove the dashboard, just the radio.
Have fun!