Experts... I need help

ok so i installed a system to my friends maxima.  I have a Kicker Comp VR12,   RMS 400 Max 800.    Running it I have a Rockford Fosgate 500S,  500 W X 1 @ 4 Ohms Bridged RMS.  Now im pretty sure im suppose to be wwithin 25 watts for amp-to-sub but i figure dont turn it up to loud or keep level turned down, whatever.   Now I used 8 guage wire.

Problem:  Lights USED to dim when sub hit or peaked, BUT

MAJOR PROBLEM... sub suddenly faded out to a very wierd sound(like it blew)  and fuse on amp wire kit blew.     I replaced fuse and tried again but  this this time the sub made no noise and the fuse blew again? I think its the 8gauge but how can I be sure i havent blown the sub or worse fried my amp?  HELP ME with all my probelms

You'll need a multimeter that can test for resistance.  Test the positive and negative speaker leads on the subwoofer (without it hooked up to the amp) and you should get something close to 4 ohms.  Anything less than 1 ohm and it's probably blown.  Also, you can try disconnecting the speaker wires from the amp, and turning on the system and seeing if the power cable fuse still blows.

Definitely though the first thing to check would be that the sub is not blown, using the method I described.