FITTING AN OEM WIDESCREEN SATELLITE NAVIGATION SYSTEM
IN A NON-NAV LEXUS GS

All pins are numbered as though you're looking at the pinholes, with the wires leading away from you on the other side.  The plugs with a green background (M2, M3, M6, N3 and N4) are the five you'll need to purchase, the rest are already on the car.  

A16
Connects to air conditioning controls on non-nav GS, or air conditioning control module on nav GS.
A24
Connects to ABS/TRC/VSC ECU, you'll need to find it if connecting the "REV" wire from the navigation unit.
C10
Connects to digital clock at top of centre console.
C11
Connects to rear of combination meter (instrument cluster).
C12
Connects to rear of combination meter (instrument cluster).
D2
OBD-II diagnostic port, in the driver's side footwell just below the boot release button.
G3
Hidden up behind the diagnostic port, this is where the gateway ECU plugs in.  If you have radio controls on the steering wheel the ECU may already be there, if not you'll need to get one otherwise you can't control the air conditioning from the navigation screen.
M2
Connects to the touch screen.
M3
Connects to the touch screen.
M6 / N3
M6 connects to the touch screen. N3 connects to the navigation unit in the boot. They are identical and connected together.
N4
Connects to the navigation unit in the boot.
P3
Plugs into the parking brake switch, you'll need to find it if connecting the "PKB" wire from the touch screen.
R2
Connects to radio.
R3
Connects to radio.  Waste of a plug really, only two pins are used!
S10
Connects to non-Mark Levinson amplifier behind the glovebox. Some GS models don't match up with the Lexus wiring diagrams, having the new radio wiring (with pins 9 and 10 used on R3)  with the old amp (with four plugs).  If yours is like this (mine is) then this is the plug you're after.