This is probably a bust; these tilt more towards 'aware' than 'human', and odds are you're read them already, but just in case:

http://www.fanfiction.net/s/3310149/1/Lady_in_Black
http://www.fanfiction.net/s/3381731/1/Queen_of_the_Road
http://www.fanfiction.net/s/3570179/1/Road_Song
http://www.fanfiction.net/s/4558154/1/Carrying_On

Thank you so much!! No, I don't think I've read those. Most of what I've read has been on livejournal. I find it difficult to search for fics with Metallicar in them on fanfiction because apparently ff.net doesn't recognize her as a character, which, as fans, we know is not true.

I was actually considering writing a fic just like the one you've described, with a slight deviation. See, I decided to make the impala a host to the consciousness of a random girl caught in the crossfire of a messed up spell. Now she accompanies the brothers as the impala, because her parents pulled the plug on her comatose body before the boys could stop them. I was thinking of setting this somewhere in season 2 or 3, keeping her as the impala until castiel shows up and decides to give her a new body.

It was heading in the direction of a dean/oc/sam, but without wincest; basically, the boys share her. (it's less sick than it sounds, I swear!) however, I hit a couple of roadblocks, and I can't seem to get around them. Sorry.

Though now that I know someone's looking for this kind of fic, maybe I'll give it another try!

Absolutely yes! Keep me updated! That sounds very interesting! :D