This may sound dumb, but check all your wires, my buddy had this happen to him, except his HU didnt catch fire, the amp did. He got pissed off, and pulled his entire system out, wires and all. When he did he found 1) the fuse melted 2) the power wire had a small knick in it and it was touching the chassis 3) he was using too big of wire for his amp, he had to force it in the amp
just a thought, good luck man.