Sounds like a dead short to ground on a fairly large wire. Check the wires running to the positive battery terminal, the alternator, any other larger wires, to make sure the insulation is fully intact, and it isn't shorting itself to ground. Those fuses you're blowing aren't small, and it takes a lot of current to cause that problem. You may also want to have the battery tested while you're at it. Usually, a short is very bad for a battery.