I have read several posts asking for information on how to disable the twilight (auto on) feature on the headlights of 99 FSCs.
I wanted to do this myself so I searched and found several methods of doing it.
This is the easiest. You don't have to take the dash apart or cut any wires. Just a simple in line resistor.
Here it goes:
To disable the auto headlights you need a 1.5K ohm resistor (Radio Shack) and 2 Scotchlok Tap Connectors (Walmart).
First locate the Body Control Module (BCM) which is under the dash, below the steering column. Laying on the floor, this should be visible without removing any dash panels. You should find a purple connector (C3). Unplug it.
Locate the white and gray wires (A4 and A6). I don't remember if white is A4 and gray is A6 or vice versa. It doesn't matter anyway. BTW - there are two gray wires in the connector. The one you want is the one farthest away from the white wire.
Tap the gray wire from the plug (C3) to one end of the resistor and the white wire to the other end. You don't need to cut the wires. Make sure you tape up all wires to prevent a short. You should now have a gray wire coming from C3 tapped to one lead of the resistor and the white wire from C3 tapped to the other resistor lead *. Plug C3 back into the BCM. Start the truck and make sure the lights are off. Throw a dark towel or something over the light sensor on the dash and verify the lights stay off. Try your light switch that has probably never been used and that's it! Takes about 5 minutes.
*You can add a switch here if you want. Just connect one end of the resistor to one of the wires in the connector (doesn't matter which one) and the other end of the resistor to one side of the switch. Then connect a wire from the other side of the switch to the other wire in the connector. This will allow you to turn Twilight Mode on and off.