If I remember correctly the wire harness in the early G's was not already setup in cars without NAV. To do this is quite some work since then NAV system links into the CAN data bus to get road speed, and display info on the dash. If that is actully the case you may want to simply get an non-OEM system installed. Some to my mind are actully better. Last year I did this with my '05 G and deposited it at:-http://www.lamboweb.com/Repairs_Sect...Navigation.htm. Others have done the same thing. Drop by a car audio shop and ask questions. It's well worth doing BTW if you do a lot of driving.
Good luck.