I've been living out of my hatchback, off and on, since 2009. Now, it's more "on" as I'm a full-timer. I say living "out of" instead of "in" my car because I consider the whole of my camping area an extension of my home. So, being a car dweller is actually living quite large ... with million-dollar views!
If your back seats will lay flat for a comfortable bed, or you can remove the seat(s) and build a good bed, then that's the biggest requirement for a livable space. Another necessity for me is a comfortable sitting area. In my car, that is the back seat, behind the driver, next to my bed. This space is used for relaxing, getting out of the weather, an office, where I change my clothes, and for a bathroom. My bed becomes a desk or a side table at those times.
The other necessity is good storage. Because space is at a premium, I use soft-sided bags because you can mold them to fit in the odd-shaped storage areas of a car. Also, when they are empty, they take up no space (unlike drawers, crates, or bins). The old adage of "a place for everything, and everything in its place" has become my mantra. Believe it or not, it's easier to lose something in the small space of my car, than it was in my sticks-and-bricks. So, I really try to put everything away in its designated spot immediately after I finish using it.
If interested, I have several how-to articles on my blog. They are in the right-hand column.
https://SuanneOnline.blogspot.com.
Have fun planning and getting ready!
Suanne