My money is with the whack from the mail box, possibly loose wire at ammeter?
I'm going to agree with Cafeman on this one. I had something similar recently, it turned out to be a red wire in the nacelle rubbing the insulation off the wire and shorting to the inside of the nacelle. Yours could be that or some other pinched location. I also would inspect the ammeter wires, see if they are securely fitted.
I'm going to guess that "nacelle" means the metal shell that houses the headlight, ammeter, speedo and all the wires below my key? Just havent seen that word used before.
For the hell of it - tried a 20A fuse - it blew too. haha. Thank you Ace... the search continues.