Cas insists on getting out of the car because he thinks his death could harm or kill the Winchesters and in desperation, knowing Cas is dying nad is terrified, and gone blind, Dean kisses him and blacks out.
Dean wakes up in a hotel room, to find Cas is alive, though still very weak, and that he's grown wings. His soul and Cas' grace have mingled and they have efectively bonded, and are a part of each other now. He learns to make them invisible and adjust his stance to their weight etc and I believe cas and Dean start a relationship.
I'm afraid I cannot remember more than that, and i've tried looking through some Winged!Dean lists but so far no luck!