hey guys i need help with my sub+amp install. i got it installed like 2 weeks ago. i noticed that when i turn up the volume the sub stops. when i am at volume 15 it hits pretty hard and all but once i turn it up to 20 it just stops working. my friend says i have a bad ground but ya i just screwed it on to the metal piece in the trunk like some beam. what do you guys think it can be? thanks for the help