Hedlight switch failure is not uncommon. 1998s had a recall
on it. Call the dealer and give them the VIN number to
see if your car was under the recall (only limited
number of 98s were). They can also tell you if it
had been fixed since they record that for recalls on
a national database.

Even though there was a recall many others who were not
on the recall list report they had the same failure.

You will probably have to get it from a dealer. Use an online
parts discounter. often their prices are very close to an
after-market source. An example is www.volvoparts.com (you
will have to call them for the switch since they won;t have
it listed on their page). I have not heard of anyone getting
the switch that was not a volvo part (this is common for
many Volvo parts since there are not a lot of after-market
suppliers except for high wear items like brake parts).