Sterios don't make it go, so in my book they are about useless.
I mean, you have something to make sure your not alone with your own thoughts (Kenwood deck), and you do have a 'baby talk to me' thing under your right foot. What more do you need??
Like there isn't some place to put your $$ to make her go, stop, or turn better??
If not, your done!
Back to the sterio question, useless as it is, do you really need a back seat?? The only reason I have found for a back seat is to cart the ... uh ... second chick home for ... Uh ... Never Mind.
Rid yourself of the back part of the back seat and install some press board and mount some real speakers (assuming your car is a notch back).
The ... extra female ... can sit between the extra speakers and use her ... Uh ... Whatever ... to hold onto and suck down to the hump. But that's the point, isn't it??