When I turn on the rear defrost of a 1999 Olds Alero, the light comes on but the rear window does not defrost. What can/is causing this?

Inspect the rear window and check the heating cords fixed to the glass. check that none of the the element cords are scratched through or broken this will stop the flow or current, also see if you cna see any cables connected ususlly there are two. one on each side. ensure the cables are connected securly.
if possible removed the cables and test for voltage at this point if voltage is present when switched on then the window element is damaged if not present then there is a problem with your rear window heating circuit. or switch.

You should take a test light & check for continuity between the feel & ground at the window. If there is no power check the switch as it is probibably burnt out.