To obtain a hard ON & OFF function from the momentary-action FWL switch contacts requires a latching relay circuit.

Here's a mechanically-latching relay I obtained from Digi-Key. It's an OMRON MY2K-US-DC12 (Digi-Key Z2056-ND). It cost $14.20 and is quite useful for many 12 volt circuits that are controlled by switches with momentary contacts. The DPDT contacts can handle at least 3 amps. The latching and unlatching coils draw only 60 ma. and 120 ma. at 14 volts. They draw current only while the FWL switch toggle is being pressed. The relay then REMAINS in the position it was toggled to, drawing no current. The image shows part numbers for a relay socket and retaining clip for mounting the relay on a metal bracket or chassis. A 25 mm. x 22 mm. cutout is required. The socket cost $1.36 and the retaining clip $1.38, locally. Radio Shack doesn’t carry equivalent hardware. The socket could be eliminated, because the relay's pins have small holes in their tips for securing wires. If you're skilled in basic soldering, you should have no trouble soldering connecting wiring ends onto the pins. Use short lengths of heat-shrink tubing for insulating the soldered connections. I would think that the relay, without a socket, could be mounted near the FWL switch using Velcro, or double adhesive-backed foam tape. I've run some tests with the relay and it seems extremely immune to vibration. I've included a suggested relay hook-up diagram below. It includes connections for the FWL switch's nite illumination LED. The amber annunciator LED in the FWL switch will illuminate when the relay is in the RESET (OFF) position. This is the relay position that will remove power from your device.

The "consumer-switched-off" bus coming from the alarm C.U. provides switched +12 volts to the rear window defogger switch on a RED/BLACK wire. That bus powers all the interior lights and is handy to tap into for powering the FWL switch LED and the latching relay coils. The bus goes dead 10 minutes after locking the car, or after 2 hours, if you don't lock-up. Another option is to tap switched power from the GREEN/BLACK wire of the telephone prep. connector under the forward, center console. You might want to do this if you're concerned about the amber LED in the FWL switch remaining illuminated for awhile after leaving the car. The LED draws a negligible 20 to 25 ma. If your device draws a large amount of current, the +12 volts input that is wired to relay contact 11 may need to be obtained from a separate, fused, ignition switch controlled source, such as the telephone prep. connector (GREEN/BLACK, 7.5 a.), instead of the RED/BLACK source.