There is actually a TSB about that. Oddly enough it involves plastic ties
The idea is to basically remove the thing, unscrew the two long narrow things that go towards the back of the vehicle, and take them off, then put the liner back against the back seat and tie with two ties through the holes formerly occupied by the screws holding the long pieces. TSb # 00-080660003A rear Seat Back Trim Loose/Sagging.
they give two solutions- the one above is for 2000-2001, and the second one mentioned by markwb for 2002-2003- reglue the magnets