How to address a person in a letter? For example, if a person holds not just "Dr." but "Priv.-Doz. Dr." or "Prof. Dr." should we include the full title after "Dear ..."?  

Thank you.

P.S.
July 20, 2014 UPDATE: as some repliers correctly guessed I faced the question when I was writing an email to a German person.
When in doubt, use the full title. Err on the side of formality, and let them correct you if they wish.