I fried my after market head unit trying to hook up aftermarket amp what did I do wrong? Please help

 

You did it wrong

 

Well it was working but my subs weren't hittn very hard so I was just adjusting gain on amp then it fried

 

You did it wrong.
Seriously though, there could be so many possibilities but without seeing it or knowing any details it's hard to say. You sure it's blown or is the fuse in the back of the HU blown? Are you running multiple amps?