Dean wasn't sure exactly how it had happened. Apparently, being a Knight of Hell meant no more driving. Sammy didn't want to come, didn't want to be stuck in the Impala with an Angel and a Demon who insisted on keeping up the work even though both were flagging. So that left Cas as a driver. And since driver picks the music…
"Cas, what the hell is this? You're gonna kill Baby," He ran his hand comfortingly over the dashboard.
"Relax, Dean. I enjoy… these vibrations."
Pop music. Cas liked pop music. Dean just shook his head.
"I will smoke out," he threatened, but it was an empty promise- he'd put too many good burgers into this body to quit it now. Cas just chuckled darkly and turned the radio up. Dean grumbled to himself for a moment more before turning his attention to Cas. It was strange to be able to see the True Form Castiel had always been going on about- the Grace was flickering out, but it was still nearly blinding.
Abruptly, Cas laughed.
"What?" Dean asked, smile curling the corners of his mouth at the familiar sound of Cas' laughter. He was proud that he could smile against the rage and kill drive emanating from the Mark, but there were some things that just stood no chance against Cas' laugh, which was rare enough to have a powerful effect.
"It's us," he gestured to the radio. "'Got my bad baby by my heavenly side,'" He looked sideways at Dean with a tiny, shit-eating grin. Maybe the world was going to hell, maybe Dean was already there, maybe Cas' Grace was going to burn him out, but right then… the angel's smirk was only a little sad. He was making pop culture jokes about shitty music on the radio.
Dean took one of Cas' hands gently off the wheel to hold, ignoring the hatred and pain stemming from the Mark when he touched his angel.
"'I know if I go, I'll die happy tonight,'" Cas finished the verse.
Dean scowled, squeezing Castiel's hand tightly. "We'll find a way to recharge your batteries, buddy." He tried to sound offhand.
"I don't think we will, Dean," He was quiet for a long moment. "Where do you think angels go now? When we die? Purgatory?"
"Heaven." Dean answered the question without hesitation. He was Crowley's little Prince of Hell, and the word left a bad taste in his mouth. "But you're not going yet, Cas, you're not leaving." Not leaving me.
Cas didn't answer.