More from the Impala 'verse, as seen in Lady in Black, Queen of the Road, and Road Song.
Spoilers for ep 4.1 and the end of season 3.
She knew the end was near. Not because of the calendar—cars are as bad with time as they are with names—but because Dean told her. Late one night at the junkyard, after yet another round of shouting that carried through the night air and would have wakened the neighbors if the junk man had any, Dean stormed out of the house and slid behind her wheel and just talked to her for hours, until Sammy came searching for him.
He told her everything, without excuse or sarcasm, told her that there was no hope, that nothing could save him, that she would have to take care of Sammy. A woman would have argued, would have railed and cried and screamed—but all she has ever been is a car, a collection of parts held together by a soul that most humans can't even recognize, and all she could do was sit there, treasuring his presence, and silently promising with all her metal heart that she would do whatever he asked of her.
And, just like he said it would, the end came.
It happened indoors, away from her and any help she might have offered, but she knew exactly when it happened—knew before the demons scattered into the night, before the junk man went in, before he and Sammy finally carried out the body wrapped in bloody sheets and carefully laid it in her back seat, exactly where they had placed dead Sammy just a year ago.
Sammy came back. But Dean wouldn't. He'd told her so. And so did Sammy, when he and the junk man had buried Dean and the junk man had driven off to his junkyard. But where Dean had told her so that she wouldn't be surprised, so that she could fight the breaking of the bond, Sammy told her desperately, brokenly, appealing to her as the only other who could really understand what it meant to miss Dean.
She knew then that Sammy had gone a little mad, the way John did after Mary died, the way she should have. For the next few days he drove so angrily and recklessly that he nearly killed the both of them before she took control and left him sitting there, torn between anger and confusion and grief. She took control a lot in those first few weeks; when he wasn't crying he was drunk, and when he wasn't either he was...frightening.
After she ate his last tape in protest of his taste in music, he hooked up some little white gadget through the lighter, which he knows full well she can't short out without wrecking the whole electrical system. But sometimes, when he's thinking of Dean, he puts in one of Dean's old tapes, or forwards to some of Dean's music on the little white gadget, and they both ride along contemplating what they've lost.
The road no longer sings beneath her wheels, and the sound of her engine has dulled. Old repairs are failing—welds giving way, scratches reappearing, screws working loose. All the work that Dean did, putting his heart and soul into it, was held together by more than skill.
Dean's love sustained her.
Sammy's grief is killing her.
Sammy's only loved two people in his life: Dean and Jessica. He cherishes the image of his mother, but even a car knows you can't love someone you can't remember, and there was too much ill-will between him and John for that level of emotion. Respect, maybe, and awe, but not love.
And she was never anything to him but a car—a special car, yes, but just a car, something inhuman. He keeps her only because Dean asked him to, and she doesn't know how much longer that promise will hold him. After the last spike in gas prices, she's pretty sure the only reason he hasn't taken her to the junkyard for storage is because that means facing the junk man, and this half-mad Sammy doesn't seek out hunters, even hunters who care as much about him as the junk man.
He's become the worst of Dean. Dean went through women at a much faster pace, of course, but they were real to him, and he gave every one his best and his full attention at that moment, if not his heart. Sammy, though, Sammy has become terrible; he's not picky at all, he forgets their names while he's still with them, and the lust never, ever reaches his eyes. The sensitivity that earned him so much teasing through the years is gone, leaving behind a shell who's desperately seeking only to feel anything, just for a moment, and who doesn't know any other way to find it. Sammy uses them and then discards them, in a way that Dean never would—never could.
What's more, some of the girls are tainted, the way Sammy is, the way he has been since Dean resurrected him. The taint had finally started to fade to a point where she could ignore it, but after Dean died it returned full force, and she can only bear it because he's Sammy and Dean asked her to take care of him. The girls are another matter entirely; she has enough strength yet to stop her engine and blare static and refuse to budge when he invites a tainted girl in.
It's never enough to stop them, just to get them out of the car. Sammy doesn't seem surprised that she won't let them in, either; he just has the tainted girls meet him at the next motel, even if it's miles away, which worries her, but she has so many worries now...
It's been four months now, and she's sunk into a kind of sleep, the only way to keep her hold on her frame, to keep from slipping into madness or whatever kind of death a car can suffer. She has to save her energy for when Sammy really needs help—a burst of speed chasing a demon fleeing in an overbuilt Mustang, one night; on another, a refusal to shift gears or apply the brakes when she can tell he's going to try to pick up a hooker. She sits in the parking lot at a motel, where Sammy has a room with yet another tainted girl, so focused on staying here that she can't keep an eye out for familiar cars, for enemies, for the telltales of demon approach.
A hand runs along her metal, bringing with it a surge of warmth and energy across her skin like a hot rain in high summer, and she thinks, for a long second, that she has finally succumbed to madness. Only one person feels that way. Only one. And he's gone, and he's not coming back, Sammy said so, Dean said so...
"Hey, sweetheart," says that familiar voice, "you miss me?" An equally familiar weight settles into the front seat. There's no taint to him, not like Sammy; he's pure and real and it's her Dean—
They drive out onto the highway, and the asphalt is pure music beneath her tires and her engine hums happily and there is nothing, nothing, that can eclipse her joy.