I'd personally say "la porte arrière" for the rear door of a car but "la porte de derrière" for the rear door of a house for instance. I think the distinction is that on a car both sets of doors are side-by-side whereas in a house they're in opposition to one-another. That's my France's French take at least.
"la porte en arriere" or "la porte d'en arriere" sounds French-Canadian.
Well how about that - today I learned people outside Canada don't say it the same way!