knockouts should be thru back of retrofit fan box
 

a fan box. i classify it as retrofit, though of course they're used when a fan is installed in
new construction.

old box (1950's) offered knockouts on sides and back, under the clamp.
http://img16.imageshack.us/img16/384...box42779me.jpg

new box, with workaround: short twine taped to pull cable into box after box is recessed
"behind" sheetrock.
http://img11.imageshack.us/img11/158...old42859kj.jpg