Press the following combination to get to the service menu of your Lexus or Prius navigation system.
Newer vehicles (07+): Menu, Volume, top left, bottom left, top left, bottom left
Older vehicles (I can confirm this one): Menu, System, top left, bottom left, top left, bottom left, bottom right
Older vehicles (alternate version): Menu, DVD, top left, bottom left, top left, bottom left, bottom right
Disable Navigation Lockout
On the service menu screen there are a number of diagnostic buttons hiding untold wonders. One of the more useful is the GPS button which gives you detailed lat/lon and status information from the GPS module. To disable the lockout feature, look for the button that says Override. Press and hold it until it beeps, then press the back button to exit the service menu.