1996 Cadillac SRX - STS Theft System Problem Car may not St

I just bought a 96 Cadillac STS. The seller said all it needed was a new battery and to have the computer reset. I replaced the battery and it cranked up but it died after a few seconds. Then I was told that I needed to replace the ignition switch and key because a wire inside was broken. I had that replaced and the car started and ran great, other than the Theft Problem Message. And no radio, AC, fuel or temp gauge. When I got it home I started it up multiple times in the driveway. But it still had the message saying " Theft System Problem. " So put the key in accessory position and I waited awhile.I was told to wait 10 mins. While waiting the radio came on and the AC lights were on but no air blowing and the fuel and trunk switches worked. Wierd. I read that unhooking the battery for an hour would reset the computer so I tried that but now it started doing the same thing as before. Shutting down after a few seconds and wont run. It acts like its possesed. Some things working then they stop working. I also tried to get codes by holding the off and warmer up but got nothing. Please help. How do I reset the computer?.I need to get this car working :)