wow..im amazed it hasnt burst into flames... somewhere you have a serious short circuit... something is drawing an incredible ammount of power, esp if your alternator is testing OK... thats hundreds of amps melting wires, or frying the relays on a burned out whatever it is... and as youve changed the batteries aare you sure youve reconnected all teh terminal ends correctly.. one error is all you need! whatever it is its dangerous... batteries explode and cars catch fire... get it sorted before it happens.