So i tried to install a sub in my 06 g35 coupe and i got it working and i think the amp i was using was to over powered cause the sub could only handle 400 watts and i used a 1200 amp watt and then all my speakers stopped working. one of my friends suggested that maybe the stock amp fuse stopped working but i really dont know whats wrong can anybody help me

 

There's so many factors that could cause this and with such a vague description of the install, it could be about 100 different things.

Find somebody who gets paid to work on car audio to check it out, otherwise, you're going to need pictures of every bit of the installation if you expect us to be able to help you.