Remove the door panel. Some Lesabre's in that period had speakers screwed in, while some had them riveted in to the door. If screwed in, unscrew. If riveted in, drill out the rivets, making sure to use the smallest drill bit that will still get the job done. This is to be sure you can reuse the screw holes if you're installing new speakers in the same location.
The rear speakers are under that "shelf" in the back where the 3rd brake light is housed. 1st, get into the trunk and look up under where the 3rd brake light is. There will be a "trap door" that you'll have to open in order to get the brake light off of the the shelf. After that, you'll then pop the back seats out of place. Then, pull on the front lip of the shelf, make sure to take out the seat belts. then, CAREFULLY, slid the shelf out, 2 people can do this easier, btw. Then you'll have access to the speakers. They are held in with 4 (10mm?) bolts that can be taken out easily with a ratchet. I hope this helps!!