I have a90 seville cadillac and the theft deterrent system has come on somehow. i have done all the manual said to do and yet it continues to activate again and again and will not let me start the car. what can i do. the cadillac dealers say that they do not work on a car this old. what can i do? please help

Optional Information:
1990 Cadillac seville

Already Tried:
lock,unlock doors. disconnect battery