I have the same car and apparently the same problem. I have tried another used switch which worked perfectly for 2 days then quit pretty much, works very rarely now. I was told by a longtime Volvo mech that there's a relay on the controller circuit board that can't handle the heat that goes through it, and new controllers are improved in this area. I am working with Uncle Olaf, in a thread near this one about speedo's, to try to figure this out and maybe solder in a new relay. Maybe stay in touch and we can see how to fix this problem.