sounds like it could be a few things, firstly the earth id say, earthing it where a plastic gromet was is no good, dont forget the amount of power going down ure earth is very high, even if u get an ok earth as soon as you turn it up it will cut out or blow a fuse! id earth it to a seat belt bolt or similar a big bolt where theres no paint and clean metal with a good conection!
if this doesent work it could well be that the sub is running to high and cant handle the power ie: going into pertection mode so cutting in and out untill the fuse blows to stop it harming itself.
personally id start with finding a good earth, turning the amp off max if it is and then running it in, if still no joy check ure conections on to the amp as if power is jumping to neg terminal when its moving around the boot then this will blow it for shore!
if still nothing return it to halfords they normally have a way of testing them in store.
hope that helps
luke
awesome audio
