When Dean brings back Fedora, Impala is not impressed. She doesn't like the brash, annoying, sexist relic of Dean's time in the 40s. He is always with Dean; always taking her time with her boy.
Impala has told Fedora she doesn't like him. When Dean leaves Fedora alone with her, she will either stubbornly ignore him to avoid hearing him laugh at her again or she will rant at him about how low his IQ is (well, really, he's a hat and she's a car, there's no contest, she will always be smarter).
The problem is Fedora knows Impala is jealous and he always wants to talk about it. He tries to tell her he knows that she is always going to be number one in Dean's heart and that he isn't trying to impose. In fact, he's willing to just tag along for the ride and be part of some kind of committed threesome.
He will say all kinds of things she doesn't want to hear because, well, the thing is Impala doesn't like sharing Dean with anyone - except for Sammy but she has a special place in her heart for Sam because she watched him grow up. She also liked that nice angel fellow, even if he was a little stiff for her taste. But Fedora? She hates sharing Dean with Fedora.
And one day, she decides it's time for the ugly brown bastard to go.
And thus begins Impala's aptly named Operation Kill Fedora.
She does things like randomly flipping open her hood when Dean takes off Fedora and rests him on the hood and then attempting to drive over Fedora before Dean can save him. Or slamming her doors closed a fraction of a second faster than normal in an attempt to rip the blasted hat off Dean's head. So many different things just to get rid of that fucking hat. But nothing works.
Finally Dean catches onto her jealousy and her murderous intent. He leaves Fedora with Sammy and sits in the driver's seat and takes Impala for a long drive, just the two of them, until they're out of the city and under the stars together, where she knows they belong. It's the kind of drive he hasn't taken her on since long before Fedora showed up and she can't help but add a little extra bounce to her wheels as they speed over the empty highway.
When he eases her off the road into an empty field and parks her, she creaks in excitment at the prospect of being alone with her Dean for the night. Nothing but empty land, her Dean, and the cool night air. It's Impala's favorite pasttime next to driving her Dean and his Sammy all over the country through back roads few cars ever get the pleasure of seeing. After a long moment, Dean gets out and goes to sit on her hood. He holds a cold beer in one hand and rests the warmth of his other hand against her cooling hood. She longs to press up into his hot touch but doesn’t out of fear of knocking her Dean off of her.
They sit in silence for a while, Dean just staring up at the stars and taking slow sips of his beer until he finally hangs his head and sighs. He strokes her hood gently and speaks in a voice so quiet, Impala has to strain to hear it. He whispers words of love and reassurance; strokes promises that Fedora will never come between her and Dean into her metallic skin. Dean explains that Fedora is more like the best friend he takes to the bar on the weekend and how, no matter what, she will always be his girl and no one will ever replace her.
Tears of windshield wiping fluid wet her windshield and Dean gets back in the car to start the wipers and clean her off. He pets her dashboard and puts on an old Metallica tape, softly crooning her favorite song. When he's done, she purrs gently under him, telling him she understands and she won't try to kill his Fedora anymore.
Dean smiles and murmurs, "that's my girl," before he starts her up again and drives them back to the parking lot of the run down hotel they are staying at this week. And this time, when Dean leaves her and eventually returns with Sammy and Fedora, she doesn't feel white-hot with jealousy, instead, she gives Fedora a shy smile and runs like a dream under Dean's steady, loving hands.
(And they lived happily ever after)