Hi,
The Volvo part numbers for the two bulb sizes are 9148906 for the longer 17mm one and 9148908 for the shorter 9mm one. Most of the switches take the longer one, but as I remember, at least the dash board dimmer rheostat takes the shorter and maybe some others do too. The shorter one is used exclusively in the radio faces such as the SC-816, etc They are covered with a tiny blue plastic boot which you take off the old bulb. My bulbs cost about $4.50 each at a dealer in the San Francisco area, but as others have said, Radio Shack may have, at least the longer one, also at less cost.
Good luck, Brad