There isn't a single "anti-theft fuse" in a 1991 Cadillac DeVille. The anti-theft system (likely the Passlock system in that year) doesn't rely on a single fuse for its entire operation. Problems with the anti-theft system are usually related to the Passlock module itself, the ignition switch, or the wiring between them. A blown fuse might prevent *some* related functions from working (like the alarm or some aspects of the immobilizer), but it wouldn't be a dedicated "anti-theft" fuse.
If you're experiencing anti-theft problems, you'll need to consult a wiring diagram for your specific vehicle or a Cadillac repair manual. Focusing on fuses won't necessarily solve the problem. A mechanic familiar with these systems would be better equipped to diagnose the issue.
