Threw together a proper perspectiv picture for you matey. Going off the 2 main flats on the car with corresponding twin parts, ie: front (2 lights, 2 vents, top of hood, top of windows) and the side of the car: (top of rims, bottom of rims) combined, you can find the horizon in whatever background you put the car in. in other words, if you placed the car outdoors, the horizon would be the blue line, but seeing as youre going for an indoor shot, if you add a rear wall, where the wall joins the ground should be parallel to the blue line. if the wall/floor join is directly on the blue line then its a pretty damn big showroom you've got the car sat in
where the wall joins the floor should be underneath the blue line at all costs, never above it. here you go: (big pic i know)
http://img709.imageshack.us/img709/7605/jhuiperspective.jpg