Here's some info regarding the computer wire connection to be used for the extended idle setup:
EXTENDED IDLE SWITCH TEST
OPTIONAL POLICE PACKAGE ONLY
The extended idle switch is used to raise engine idle speed to approximately 1000 rpm when the auto shifter is in either Park or Neutral position. A rocker-type switch (extended idle switch) is mounted to instrument panel. This switch is available only with 4.0L engine when supplied with optional police package.
The extended idle switch will control a ground circuit going to the powertrain control module (PCM) When a ground signal (through the switch) has been received at pin/cavity A-12 (circuit K78) of the PCM, engine idle speed will increase.
For 1984-1992 I6 XJs and all I4 XJs:
Not an option in computer pin-out.
For 1993-1995 I6 XJs:
PCM Pin # 10, Purple, [60-pin connector] (Switch On: 1V, Off:12V)
For 1996 I6 XJs:
PCM Pin # 12, dark blue/orange, [Black "A" 32-pin connector] (Switch On: 1V, Off:12V)
For 1997-2000 I6 XJs:
PCM Pin # 12, gray, [Black "A" 32-pin connector] (Switch On: 1V, Off:12V)
Switch location for 97+ (optional 4 opening switch bezel)