Hi Huron,
For my particular setup, I didn't have to fool the nav, just the tv tuner which now controlled the video output to the existing Nav Screen. Power wise, the tuner came with a positive, negative and parking break wire. The positive goes to the battery and the negative to a bolt on the chassis as usual. Instead of connecting the parking brake wire to the parking brake, I just grounded it (same place as negative was done). IMHO, there's two things to be careful about by doing this,
1. The tuner is turned on/off by the remote control, so one could leave it on after turning off the car and drain the battery (though you'd notice cause the screen would still be on). Possible workaround, hook the "Parking Brake" wire to the power antenna negative (which is right there on the right side) so the tuner is getting 'juice' only when the radio is on (need the radio for the tuner audio anyway).
2. Never heard of anyone getting pulled over for this, but it is illegal to have a tv viewable by the driver. Then again so is speeding...