Well, to me, an affidavit of defense is different than a revocation letter. In Illinois if a creditor initiates a wage assignment, they are required to provide an affidavit of defense that the customer can fill out if the have a valid defense (ie the account is not theirs, is a valid defense).
A wage revocation letter just means you are revoking your wage assignment, it's not really a defense. But it doesn't have to say much. You can state "Dear XXXX Company, I hereby revoke the wage assignment I signed on xx/xx/xxxx." That is just as effective as if you quote all sorts of laws and what not.
There are letters here on the site. You can go to the search box in the upper right-hand corner and try searching for "wage revocation letter" or "wage assignment revoke" and you should get a bunch of variations.