Let me start by saying that I am not a 46 guy, but based on the wiring diagram on Mac Van Pelt’s website I would say it definitely does not go in the headliner. Attached is a link to this diagram. It looks like it attaches at the same location as the headlight harness, which I guess is in the engine compartment. Then, I think the harness would route along the inside of the frame rail, similar to my 1937.
http://www.vanpeltsales.com/FH_web/F...1946-48car.jpg
I will let the real 1946-48 guys correct my mistakes.