Well the unit right behind the battery is the SOLENOID (damn me and my decent spelling skills ) Sometimes they will kinda weld themselves in the "on" position which would put a huge drain on the battery causing lots of sparks. A new SOLENOID costs about $8 at any auto parts store. May want to drop one in and see if the problem still persists. The problem with hunting down fusible links is that they look like normal wires on the outside but may be burnt on the inside . I'd swap the SOLENOID first and see if that helps.