On the '80 this is fuse #6, this fuse also is the one for the interior light - so if your interior light also is not working it would be the fuse. Good to carry a set of spare fuses in the glove box; IPD carries a 30 piece set of these ceramic fuses for under $5.00. If you do not know about IPD, check them out on their website at ipdusa.com Request their catalog for the 200/700/900 series, lots of good stuff in there and tech tips. They also will send you a mini-catalog newsletter several times a year that also has some very good tech tips.
You might check out servicing/cleaning the fuse box on this site. Over time the metal brackets holding the fuses can loosen and need a "pinch"; if the car has spent a lot of time sitting out in the damp the tips of the fuses may need cleaning or some contact grease. Went through this with my '92 last winter when a leaking windshield routed the water down through the fuse box area! Over the next few months I had some fuse related problems, the fuse was not broken, but was not working properly, switching in brand new fuses solved the problem.

Good luck and welcome to 240's and Volvos

245 Janet