Dear cox084--  The wiper motor for your 1990 Chevrolet Lumina is in the engine compartment, just in front of the windshield in the area of the wipers themselves.  For diagrams and text, go to--  http://autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/0e/d8/5e/0900823d800ed85e.jsp.  That will show you how to get access to it.  Click on the wiper motor link for instructions on how to remove and replace it.   Good luck from Bovan.