In reply to SARS:
I've kipped in the car for a few years now and have picked up a few simple tricks along the way that have made the whole experience a lot more bearable.
1: Park uphill where ever possible, that way when the front seats go down they are pretty much horizontal.
2: Carry a couple of small pillows, one goes at the lumber region (where the seat and the back meet thus smoothing out that bump) and the other for you to put under your head.
3: Stick a semi-inflated thermarest under your feet/behind your calves, supports your feet for a more comfortable sleep.
4: Wear a dark (but thin) beanie, saves all the hassle of trying to black out your car windows, plus it stops your car looking like a wannabe drug dealer...
(Or you could get one of those eye-masks you get on planes but at least the beanie could come in handy if you suddenly feel the need to go bouldering when you wake up!)
5: Sleep with the windows open a bit but be aware of wind direction/possible rainfall through the night, where possible you want a decent flow of air to help prevent too much condensation, however its also considered a bit silly to wake up to a new water feature in the foot well.
6: If your going to car-kip in summer, in Scotland, take a flame thrower, you'll wake up in the morning to a car full of vampiric, winged b*****d beasties!
7: Normally I'd advocate a few beers/nippy sweeties just to help you nod off, never had any hassles with it personally but do appreciate what's been said above, however usually when I'm kippin in the car I'm in the arse end of nowhere.
8: Never mind all the fancy electric stuff, though an inverter might be useful, personally I've got by fine on the CIG lighter port with a USB charger adapter, a wee head torch and a decent book. After all the reason you go away is to get away from the rat race no?
9: If your feeling pretty sleepy either turn off the radio or out a soft countdown timer on your phone, getting woken at half 4 in the morning as the last morsel's of charge in your battery bleed out is not the nicest experience, especially when you're in the aforementioned, "arose end of nowhere". Though if it is gubbed you should be parked on a hill already and it'll be a matter of rolling downhill backwards and it's supposedly easier to bump start a car in reverse.
As a point of note I'm 5,4" and have car kipped in type B corsa's, old Astra's and Ford Fusion's with no hassles, though 5 door models are a lot easier to move about in than 3 door options....
Hope this helps.
Cheers,
Iain.