The whole engine gets hot if you drive long enough. You can tuck things (on older V8s with roomier engines) on top of the intake under the air cleaner housing and it will get warm enough to cook some things. You're right, putting things on top of the decorative heat shield of a modern car won't do as much -- but still, you could wrap some hot dogs in foil and they'd cook enough after a while, even in a modern car.
The canonical tome on the matter was first written 25 years ago and has been revised twice since.
https://www.google.com/search?...