The copy of a service manual I obtained from a friend said that the '91 SSE anti-theft module should be located above the left kick panel and behind the dash. In reality, it is located (at least in my '91) above the accellerator pedal. I found it by tracing the wire colors and locating the in-line fuses under the dash.
Here'* a copy of the electrical schematic --> PDF file of Anti-Theft Schematic
Here'* what it looks like under the dash... (I'll edit the size)
The anti-theft relay is zip-tied to the anti-theft module, and the ignition interrupt relay is attached to the module by a single screw. The module is held on the bracket by a single screw. A 7mm socket was used to remove both screws. Once the module was loose, it slides toward the dash and up to release from the bracket. Then I brought it toward the dash and rotated inboard and down to remove it from underneath.