So i'm guessing there was no shelf in the car when u got it? Well if u haven't got a shelf, u haven't got many options left. To get bass out of 6x9s they need to be in some sort of enclosure; when they are mounted on the shelf (as they usually are) the boot acts as the enclosure. The other option is to make (or purchase) a small enclosure designed to house the 6x9s, but u could probably also make a shelf for around the same cost.