You'll need to select the polys that make up the roof and detach them as a separate object. There's some on the underside of it too.. was a PITA when I did it for my 3DS scene
Same goes for the hardtops, but there's a but more work to do as once you've removed it, there'll be gaps between the outer skin of the car shell and the interior panels along with the possibility of needing to actually slice polys to get it to split at the right places.
How you achieve any of this in blender however, I have no idea.. but the basic principle is still the same
had a play with it and come up with my first render, basically straight out of the box as described in the tut,
happy with the results apart from the wheels and wheel arches ! they seem to be made up of straight lines,
is there a way of getting blender to calculate them more to give a better curve ?
have looked back thru this thread and noticed that some of the renders are the same as mine but others seem to have got round this problem