This is a frustrating problem. I have a Kenmore cannister vacuum circa 1987 that took me several hours to unravel the mystery of getting it open. Fortunately I have the original manual and exploded diagram, but it's still hard to figure. Sears does have the diagrams on the web now if you know the exact model number.
Anyway, the way mine works you have to get the top canopy off which is the plastic cover around the retract button, power switch and on my vacuum the overload indicator. This is tricky. You have to push out on either of the rear corners of the canopy until you can lift it up. Leave it there and then go to the front of the canopy on the same side and carefully with a screw driver pry that out about a quarter of an inch and you will be able to lift it up. Now just keep lifting on that side and you'll be able to lift it on off. After that, there are two screws near the back that hold the motor and cord winder housing on. As always, make a note of how things are attached as you start disassembling.
Bruce
November 2005