Hello and welcome to JustAnswer.
My name is Vladimir.
This procedure does not apply to your Deville. You car does not have a Theft Deterrent Module, the Instrument Panel Cluster does these functions.
You car uses a PASS-Key® II.
When a key is inserted in the ignition, the contacts located in the ignition cylinder contact a resistor pellet located in the key. The instrument cluster (IPC) measures the resistance of the key pellet. The IPC then compares this value to a value stored in the IPC's memory. If the values match, the IPC allows engine to start.
If the values do not match, the IPC does not provide ground to the starter relay and sends a request to the PCM ( engine computer) to disable fuel injectors control.
These vehicles are well known for ignition lock cylinder issues.
You should measure the resistance of your key and then make sure you get the same stable reading at the connector C202 ( terminals E12 and E13, base of the steering column) even when you turn the key through the all positions. If your reading is unstable, then replace the ignition lock cylinder.