I recently bought an hhr without the premium sound system. I was wondering could I buy the amp and sub and install it? Did know if the wiring is already there or do I have to buy something special. Cause I need the room for hauling so I can't install an aftermarket system.

 

Nope nothing is there if you have the base system, and you would need to get the premium head unit and have it programmed also.
It would cost less and be easier to go with an aftermarket system.
You can mount the subwoofer in the stock location using a custom enclosure and mount an amp beside the battery. Looks stock but sounds better.