i suppose one could connect it to get power when the ignition switch is on or off depending on where one chooses to connect it to the fuse/breaker box. where it was in 1940 IDK. i'd have to guess it was dependent on the ignition switch being in the "on" position. seeing as how radios at that time had power hungry tubes in them. not that that in and of itself would be of much benefit unless the engine was running.
in any case i clearly remember (because my dad was furious) completely running down the battery while listening to the radio (tubes) for a couple of hours in our 1961 Buick LeSabre for "Wild Thing" (the Trogs) and "Li'l Red Riding Hood" (Sam the Sham and the Pharaohs) to be played.
it probably makes little functional difference with the solid state radios of today.