It is actually the connector that is over heating due to poor contact or lack of contact and an excess amount of the current going through 1 or 2 pins beyond what they are capable of carrying. You could send over all the current with 1 positive and 1 ground if it were thick enough , but that would be a thick, bulky wire. Car stereos use 12 volt and you used to find a lot of people putting 1000w+(peak) systems in their trunk. The 8pin connector has more surface area, but was designed and specced the wires and contacts for a lower wattage. It is hard to re-spec that all these years later with all power supplies out there safely.