It's pretty obvious that whomever owned the car before you left their CD in the deck. Unless it has a name and address written on it, I can't imagine any reason why you cannot consider yourself the (now) lawful owner of this CD -- particularly since you don't have any way to return it to it's original owner.
BTW, please satisfy our cat-like curiosity and tell us what the music on the CD is? Can you identify the artists or songs by name? At least give us the musical genre (rock, pop, country, classical, etc.).
Bitter: I'm curious as to why you're not an Eric Clapton fan? He's an awsome guitarist, although I can sing most of his songs better than he does! BTW, I once tried to write a send-up of his song "Wonderful Tonight." I called it, "Miserable Tonight," but I cannot find the file with the lyrics. The first verse went something like this:
It's late in the evening
She makes me pull out my hair,
She's throwing a perfect fit
I can't tell what's eating her.
And then she asks me,
Are you picking a fight?
And I say "No, but you're sure Miserable Tonight."
...and it goes on from there. You get the idea.