Plot bunny
This is a plot bunny which has really grabbed me, but I don't feel I have the time nor skill to do it justice. It can be summed up thusly:
The Impala is a Transformer.
Enjoy.
Edit: I was thinking along the lines of that being the reason Dean has always been protected in the car. Even just a short peice from the Impala's point of view about how it's her duty to protect him, and she knows they share a deep bond, even though he doesn't know she's a transformer.