IF the defrost isn't working then it's not the servo. Defrost operates on a safety bypass of the server as a safety so that you can always have heat\see. I'm not sure by your description if your saying your getting cold air, or no air movement at all.
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS
1979 Silver 300D "The Silver Bullet" -Sold