See the page at http://www.brickboard.com/ARCHIVES/2001OCT/10085796.shtml or do a Brickboard search for: replace radio bulb.

Last month I followed these instructions to replace a burned out bulb on the SC-813 in my 98 S70. I got a package of two bulbs at Radio Shack for ~$2. It took about 30 minutes to replace the one that was burned out. Make sure you have your radio's code before you remove it from the dash. There were minor differences in my unit from the details in the instructions (# of screws to remove in particular). The design was revised a little since the 850.

My Volvo dealer told me I would have to replace the whole radio (lazy, greedy people). This site rules!