Cars that sit for that long invariably get corrosion on wire connections, terminals, switches, etc. This is true with indoor and outdoor storage.
I think you'll have to trace the wiring from the socket all the way back to the switch. Anywhere there's a joint or connection, take apart the connections and run a brass brush over them or sandpaper, etc. Make sure all the connections, including ground wires, are clean and tight.
Clean the fuse terminals, too. Contact cleaner and little brass brush works well,
