Here's a demo of this type of system being used in an Audi. There's a camera in the front grill, one in back where a regular backup camera would be mounted and one in each side mirror. On Class B vehicles, the side cameras could be attached to the body of the vehicle. The small computer that stitches all of these images together is inside the vehicle someplace.
This demo is of someone parking with a Rydeen brand system guiding them. This particular system is available from Amazon for about $635. The cameras for this system look very much like standard wide-angle backup cameras. I expect that there will be more aftermarket systems in the near future.
Below is the kind of thing you'd see on your dashboard display with one of these systems. Note that the lawn and sidewalk in the overhead view wouldn't be simulations. They'd be actual images of what's currently around the car.