will this be hard on the deck if the sub is next to the sub (with in 16")

 

in short, probably.

anything thats got moving parts isnt going to like being shaken to bits by a sub.

 

if the sub is next to the sub? I think you may want to reword this.

Also, It most probably will be harder on the deck than if it was in the front, how much harder all depends on whether you want to risk it and find out.

 

As long as your deck is sucurely attached (no vibration) to your car I don't see it affecting it so much, these units are built to take a certain amount of vibration, Sound waves aren't going to tear it apart or anything.

I'd fear a gravel road more then this anyday

 

move my sub under my dash

 

Hopefully your deck doesn't skip CD's too easily.