Walter,
I'm not sure how all of the factory boats were done, either. Our boat was a "kit", but finished by factory crews off the clock and after hours. We completely rewired our W32 in 2008
(click here for all the photos), and all of our cabin wiring was under the 3/4" tongue/groove fir headliner (only ONE light worked...) It wasn't practical for us to remove the headliner, so we cut the wires off and left them under it.
Then we ran all new wiring under the teak trip along the edges of the cabin top where the headliner butts up against the coach sides. For each leg that powers a light, we used a surface-mounted plastic wiring conduit to run out from the trip and to the light. There are a couple photos at the links below (the best I can find to show this.)
New wires coming out of trim
Wiring race surface mounted, leading from trim to light.
The benefit to doing it in this way is it's very easily to access all of the wiring and none of it is hidden beneath the headliner. We've decided not to put anything below the headliner; all of the backing plates and wiring are on the surface; easy to see and maintain.
Hope this helps somewhat!
Cheers,
Aaron N.