I just realized something.
Crowley is very protective over his car, something that represents him and his identity and his control over things.
Aziraphale gets in the car, and changes EVERYTHING about it, from the music to the horn to the color of the car to the travel sweets.
And then Crowley calls and sharply informs him, "You know I can feel that, right?!"
It's not just a car, it's him, it's CROWLEY. He's taken charge and changing everything about it that was Crowley, making it his idea of better. He literally tells the car, "There, now that's better isn't it?"
And this was after Aziraphale had said, "It's OUR car."
This is foreshadowing the end, and everything Aziraphale says to him.
"Well of course you said no, you're the bad guys." -- Of course the Bentley isn't going to want to speed, that's bad.
"I can make you an angel again, it will be just like it was before, only even nicer!" -- Oh, but Crowley, why the black when the yellow is so pretty?!
It really drives home (heh) how off-kilter everything is without Heaven and Hell. Before, they knew where the other stood. Perhaps they thought knew WHY the other stood where they did (e.g., "Well of course you're all dark and moody, you're a demon.") And now that that's gone, they suddenly don't have this backdrop.
Crowley seems to not realize that Aziraphale is silently screaming that he wants to be together, he's still stuck in the Heaven-era version of him saying "we can't really be an US". Even at the end, he tells Aziraphale, "We could have been us," and he only seemed to have barely recognized just moments before that they HAVE been an "us" for the last few years.
And Aziraphale seems to have trouble parsing the difference between what Crowley was because he was a demon, and what Crowley was because he was Crowley. At the end of the season, he's asking Crowley to hand over his metaphorical keys to join him in Heaven, so he can change everything about him in ways Crowley doesn't want.
It's THEIR relationship, but Aziraphale wants to change the terms, change everything about it so it can fit his bright and cheerful picture of how everything should be. And when Crowley angrily rejects this (just like he did when it was the car), Aziraphale is just as surprised he feels that way.
For there to truly be an "us", for the car to be theirs and not just one or the other's, they're going to have to learn who they really are on their own, and relearn who the other one really is deep down, then learn what they really want from each other (and what concessions they're willing to make) moving forward.