Unless you are playing something quadrophonic, you don't really need speakers to produce mid range and high frequencies behind your head. You can use them as woofers to fill out the sound. If you put simple but powerfull drivers in the cutouts and use low pass filters on them. Because the frequencies they produce will be low and have long wavelengths, this will not mess up the stereo imaging supplied by the front speakers. In my case I did not even cut through the covering, as it does not hinder the bass significantly. I did however cut a circle out of the white sound packing so that the cone of the speaker would not push against it. It was cheap, simple, and it doesn't show at all. Anyone looking into the car would assume that I still have the original sound system and break into someone else's car.

Oh yes, I did change out the front speakers as well. The sound is way, way better.