how do you do see through bonnets?
I'm sure Sik_VR could write up a tutorial for you but i'll give you a brief description.
First you need 2 photos of the car both a the exact same angle just one with the bonnet closed and one with it open.
Then you want to have the picture with the bonnet open on the first layer.
Then add a second layer with the picture of the bonnet closed.
Then go to your eraser tool and turn the hardness to 0% and a decent size and start slowly erasing the bonnet revealing the engine underneath.