Previous owner may have connected the hot and ground wires together with a wire nut and tucked it out of the way somewhere.
Generally most times the macerator pump is not permanently mounted as it needs to be lower than the bottom of the tanks in order to work properly. Previous owner may have added some sort of wired plug in near the tank valves and may have removed that but left the switch and rest of wiring behind.
I would just disconnect the wiring from the switch for now and then try to trace the wire back to the fuse position that blew. If you can find that wire near the fuse panel, cut, terminate it at the fuse panel and if nothing else try to remove that wire for good..