NismoS13Kings

So i picked up this s13 and the original owner told me if he leaves the battery connected for more than 2 days, the car will be dead, and that the car eventually turns over after a few turns of the key. I assumed the culprit was the aftermarket cd player he installed since that is the only electrical related mod he did to the car. Well one morning the car didn't start after connecting the battery back up and i noticed this plug, i pushed in the wires and then the car started. Id like to replace this plug but have no idea what its called or if it actually had anything to do with the car starting. Anyone have an idea?