Sadly, I've read there are many electrical problems with the 1999 S80.
In fact, Volvo dealerships are selling '99 S80s for $19k Cnd, and '00 S80s for $28k Cnd!! Why such a big difference for one year, the '99 still has a CPO warranty 'till
160kms when purchased from a dealer, and the '00s had more kms on average!
Lemon Aid (Canadian publication on new/used cars) doesn't recommend the 99 S80, nor the '98 S/V-70 for their numerous electrical problems.
I hope someone will be able to help you with your problem, however, writing to Volvo may be a good start. If you have a local car authority like we do here called the APA (Automobile Protection Agency), they may be able to help out with secret warranties and provide you with information you normally wouldn't be able to get from a dealer.