[Galane]

The way many cars used to do that was simply to have the air intake duct edge about 1/2" above the bottom of the cowling floor, then mounted a plate over the opening with the edges turned down and overlapping the raised edge a bit. With that inside the cowl area, under a slotted grille or a pop up air scoop (ram air heater!), it's very hard to get water into the intake but air flows in.

Heaters were simple, the center inlet dumped directly down to the heater core then the air flowed out the sides of the fan box. If it had the optional windshield defrosters there would be a pair of wire wound cloth tubes coming out the sides of the box, going up to pieces at the base of the windshield under the dash. On the 1948 to 1950 Hudson the defroster air flowed into a small plenum with several outlet slots on top for more even distribution.

Newer cars (1960's up) usually have the air inlets off to the sides, still with the raised edges but with solid areas of the cowling top over them so they're far enough from the grille to keep water from getting in.