Part of the answer is here: Steve H from 2005
L300 lights/tail and instument?
If you are trying to test bypassing the combo switch, from before the wiring harness begins,
you need to patch or probe power to the GW wire that extends down from the combo switch to the actual harness connector under the drivers feet. When I gave the GW wire 12V, my missing dash and running lights light up fine. So my combo switch is dirty and needs cleaning.
If I look at several of the circuit diagrams from the manual, e.g. daytime running lamp circuit or Tail Lamp, PositionLamp, License Plate diagram, or the HeadLamp Circuit, I only see the colors that are on the car side of the harness, so that it looks like GR should be given power 12V or shorted against RB.
But really, on the combo switch side of the first harness connector, that GR is a GW wire.
However, none of the circuits show the colors of the wires on the combo switch bundle side. They do not seem to get documented. On the diagram,
they make the 18-inch wire bundle that goes down from the combo switch to that first harness look like nothing at all. It shows the the A-55 harness connector as if it were right next to the combo switch,
and shows absolutely none of the wire colors in that wire bundle or on the combo switch itself.
Tomorrow I will probably pull my steering wheel off. I can try to get pictures of the combo switch and try to document the wires colors and functions in the bundle that runs down the steering column.
SUMMARY:
To do the bypass test, jump the GW (Green with white stripe) wire from the combo switch.
You can carefully use a backpin to the A-55 harness connector by your feet.
If your dash and running/tail lights lights all turn on, then the combo switch needs cleaning or replacement.