Might be able to pull out the stereo and cut some mounting holes for the pi with an appropriately sized drill bit. As for the screen maybe take some measurements and print a mount?
With that you could take the car stereo's wiring harness and connect it to the GPIO pins for power. A cursory google search shows a wiring diagram for someone who has done it. (of course take all appropriate precautions and don't automatically trust everything you read online)
