I suggest like the others.... sell it yourself.....
The dealer will cetainly not give KBB
If your mirror is missing - my guess is other visible damage exists
Plus - winter is coming..... a dealer that isn't Ford (and even Ford themselves) aren't going to want to site on a convertible... it'll get shipped to an auction and they know they'll get nothing for it there - er-go you get nothing....
Sit on the Ford until next Spring and flog it to some kid that just wants something 'cool' (erm... note the quotes

)
Cheers