Ususally it is only a single bulb but once in awhile the sockets the bubs go into get corroded and needs to be cleaned off.
other issues are as the seals on the get older water can get inside. We had a issue with that on company onwned vehicles that were given to employees for use durng work. They used to turn the Water Hose on the lights and it forced water passed the old seals.

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84 300D, 82 Volvo 244Gl Diesel