I have the canister type (Lux 2000) I pried off the back cover and found that the cover is held on with four tangs on the back cap that go on to fount small plastic posts (two top and two bottom) inside the back of the unit. From the looks of things I would bet that you are supposed to take the whole housing apart to get at this. I took a screwdriver in the center of the top wedged it in the slot between the main housing and the back cover and pried down and back at the same time. I may have been lucky as I did not brake anything. I then cleaned and lubed the return spring and the locking mechanism for the cord spool and tested it before putting it back together.
Getting it together was much harder. You will need to put the opposite end against something that will not move. Engage the bottom tangs into the cap and the with a screwdriver pry up in the housing to get the top tangs in place. I had to do one side at a time.
Hope this helps
Dean
Dean Smith
July 2005