This thread is from a Expedition/Navigator forum, but I'm sure the systems are similar. I plan on trying this on my LS and Gator very soon as not being able to have a passenger enter details while driving is idiotic.
All that you need to do according to the instructions is put a switch in to open and close the connection for the VSS while driving. The navigation system will not update position while the switch is open, but you will have full functionality of the system.
Here are the instructions:
1. Remove the black dashboard trim. Remove the two screws above the instrument cluster. You will be tempted to just bend the trim piece enough to get at the headunit screws but the headunit must come out of the dash anyway.
2. Once out (or out far enough to get at the harness), looking down at the back of the HU you will see a harness with only 4 wires on the right side. There will be a black wire with a white stripe, that's the VSS wire.
3. Push back the cloth material wrapped around the wires and cut the wire a few inches from the harness.
4. Strip the wires and crimped on two 20ga crimp connectors (the red ones).
5. Prepared your switch by soldering about a foot of wire onto each pole. I used a simple STDP switch, but I would suggest mounting a black push button switch somewhere near the HU. Not a momentary switch. You want one that will switch b/t on and off and stay either on or off.
6. Crimp the switch wires into the VSS wire crimps you place on the VSS wire in step 4.
7. Mount your switch where ever you like. Done deal!
