Okay, so I was watching this video
of Silent Hill 2, and the camera gets closer up to the car than usual. The rear window is extremely fogged up so you can't see inside well, and it's the old two-door kind of car so the seats would be the style that lean pretty far back and obstruct the rear bench seat a lot, so you couldn't see anything back there very well.
If Jimmy was as out of it as I think, he'd have plopped her in the back all willy-nilly, and considering his mental state I don't think he'd have buckled her in. If he took a sharp corner with her back there, she'd slide over and fall onto the floor of the backseat. That way, no-one on the road would see Mary's worn-out corpse, and since James wouldn't be looking in the backseat for the map he wouldn't see her by accident, either.
So, for the matter of WHERE Mary died, as opposed to where her dead-as-dead-can-be corpse was stuffed, I had assumed it was one of two things: my main assumption was he murdered her at home and drove to Silent Hill.
The other one I've had for a little while now: Mary was a terminal patient, so she had almost zero
chance of living through her "damned disease" *hackhackcoughlungcancercoughcough*. So, if she's basically waiting for death to drop on by and pick her up in his pimpin hearse, James wouldn't want her to spend the entire time at a hospital--he'd have her transferred to a Hacienda, one of those cozy rest homes for the old and/or terminal who can't really go home without kicking it, but don't want to die in a hospital. It's a series of cozy, homelike rooms with decorations the patients can bring in, like family photos and personal pillows, but the beds and wallpapers would be the same, and the floorplans might vary a little, so a bed is pointing in a different direction. It'd go a long way to explaining why there are family photos in a cookie-cutter room, and the bed is oriented differently in the Videotape and the Final Conversation.