Second the voltmeter idea. Check the 12V constant using the ground on the HU connector as your ground. Also check to see if the 12V accessory switched line turns on when the ignition is in the on position. If those have power and you've changed the fuse, then the HU is gone or the screen (if its a removable type) is not connected or gone.
On the idea of not using your ground wire: If you test the 12 V constant using the ground and you find you get nothing, check with a solid ground other than the ground wire. This will tell you if the ground wire is at fault or if the 12V lead is at fault. If the 12V is fine and the ground is bad, you won't get anything when connected to the 12V + and the ground. If the 12V lead is bad, you won't get anything no matter what you do.
For reference: red is 12V constant, black is ground and orange is 12 V accessory switched.
As for the actual "slamming" issue, check polarity of subs. By that I mean flip the polarity of one of the subs. Also try moving the subs around in your trunk. It seems having the woofers firing towards the rear of the trunk while being within a couple of inches to the back wall provides the loudest sound.
It destroys your trunk, but that makes it as loud as possible in the car (while being in the trunk). Theres a reason subs in your trunk sucks, and the best position destroying the trunk is part of it. On our cars its not as bad for trunk rattle because there is a rather large vent on the passengers side which aids in the pressure equalization.
