lost earth connection

Suffolk -the toilet flush pump will not operate as the earth return is no longer connected to earth. probably mice? There is 12v + available and temporarily taking a wire out of the cassette door and touching chassis allows the pump to run. I could install a new earth wire, however I cannot find a route for a new wire out of the toilet cassette area to the chassis with removing the whole of the toilet assembly. Has anyone else had this happen? Does anyone know where the return wire is routed and where it is connected to the chassis earth?