GKDantes is correct in that booleans are not very much of a preferred modeling technique.

I am assuming that the car body you're working with is a vertex object.

There are a few different ways that you can proceed to use booleans to give you results that are acceptable but usually they result in additional work in the final model to get it to look good that's tricky or a ton of work.

In short, build a model of the shape of the door that you want, overlap that shape onto the car body and perform a boolean subtract.

If you find that booleans are the path that you want to go, you can reply here and I'll post a few weird tips on it.

-Kix