you could pull over in a rest stop (the kind that the highway maintains) or a truck stop (which might be much brighter and noiser. if you can fold down the back seat & get into the trunk, that's a good place to sleep, (with your heads behinid the front seats and your feet extending into the trunk. maybe a layer of foam or camping mattress would help to even out the bumps.) although it requires you to move everything in the trunk to the front seat before you can get to "bed."
you may want to take towels with you (apart from the rule in the Hitchiker's Guide to the Galaxy (which, by the way, would be cool to have as books on tape for the drive. the radio show was better than either the books or the movie -- you got the different voices, but you could imagine your own scenarios and that's 'way bettter than a movie.) anyway --- towels. apart from the obvious sponge-off now and then, you can roll them up in the window as temporary blinds.& for privacy. and one of those foldout windshield thingies, too.
and just for safety's sake -- have a cell phone and call people. always have someone who, if they don't hear from you, say, in 12 hours, will know to start to worry about you, and where you were when you called them last. it's just something that we *can* do, nowadays, so it's a good idea to take advantage of the technology. especially if you're driving cross-country b/c on those last couple of days, you're going to be getting kind of loopy unless you're *really* good at sleeping in cars.
at one point, on my last long drive, my daughter kept falling asleep, i'd already memorized the music we had in the car, and no radio stations were coming in. and i'd talked to everyone i wanted to talk to. so i plugged in the headphones, called my husband, got him to put the tv on Judge Judy & leave his cellphone on top of it. worked for a good two hours, what with the people's court.following, and all.