I've slept in the car dozens of times on road trips. I am sure in some places there are no-goodniks in rest areas but I have never encountered any, but when I have done this it is usually in the middle of Nebraska or something. Nobody there but truckers and cows and most of them are not looking for trouble. Have also slept in the car because I was drunk and didn't want to drive home, and occasionally because I was too tired after playing in the casino. If you don't have a large SUV or camper pickup to sleep in the back of, it's better to sleep in the front seat and recline it, there's much more room than sideways in the back seat.
Goofiest time I ever slept in the car was when I drove out to a small airport in the middle of nowhere at night to relocate an airplane. When I got there there was freezing fog so I couldn't take off. While trying to keep the wings clear of ice long enough to take off I left the headlights on the car on so I could see. By the time I gave up the battery had run down enough that I couldn't start the car, so I had to sleep in the car parked there out on the airplane ramp, and wait for the airport manager to show up in the morning and jump the car
Temperature was about 10 degrees and of course I wasn't expecting it so no sleeping bag or anything. It was cold, but people have slept in colder, at least I was out of the wind.