Putting subs in your trunk, and ANY speakers on your rear parcel shelf, is a bad idea. The pressure generated in the trunk by the subs will affect the cone motion of the speakers in the rear deck. You are much better off leaving speakers out of the rear deck altogether, and utilizing the holes to help allow the pressure/sound waves from the subs escape the trunk and into the passenger compartment. This is especially true if your chevelle's back seat has the traditional vinyl upholstery. That's unless you fire the subs forward, and seal the trunk off from the passenger compartment. But you wont fit four 12's doing that.