I've slept in the back of a Forester and a 94 Corolla hatchback. I'm guessing you have a similar amount of space as the Forester, maybe a little longer.
Here's my advice:
Set aside one half of the back for sleeping/dry stuff. Have a Thermarest or something under your bag, and use one of the front seats as your backboard. And don't forget an extra blanket. Your car does not insulate well.
Then, you need to find some way to keep the wet on the wet side. Some kind of plastic, probably, and a small vertical partition would be ideal. Be creative.
You should also find those plastic things that allow you to keep a window cracked without rain getting in. Then you can always have ventilation so your wet shit dries better and your car doesn't end up smelling like butthole after two days.
Basically, you're going to have to figure out your own systems. It's a little different for everyone.