I don't think there's anything wrong with the amp itself. I think it's a problem with a voltage spike from being turned on via the car's accessory switch that's blowing the fuses.
If the radio is putting out a solid 3V and you can't afford another head unit, or head unit repair, you can get a relay from Radio Shack that will turn on with 3V from the head unit to put 12V through to the amp to have a delayed turn on. That will solve the on-off-on-POP! problem when you start the car.
