First, I don't recommend ported boxes in cars unless you are competing. In this example, below 42Hz, it will be very easy to excede the xmax of the drivers. I don't know what kind of music you listen to, but it's very possible if you like deep bass. Next, not only are band passes are hard to perfect with a retangle box, let alone trying to use a port size you already have.
As for sound bouncing off the rear window, bass doesn't bounce too much as the wavelengths are large. But, a port into the cabin would be a little bit louder.
I don't mean to discourage you, but it's a lot of work to get it sounding right. If your friend is willing to take the lead, go for it.
Aaron
