the reason it melts is due to high amps somewhere. Something is drawing a lot of current. I would put a multimeter on it and then go through keys in, 2nd click, start engine and see what the draw is at each stage. If it is more than 30-40A then it might need a thicker cable (like you get on In Car Entertainment amplifiers, active subwoofers etc.) a good cable should be bigger than 60amp capacity.