When defining GFX files such as this:

	SpriteType = {
		name = "GFX_HAW_armored_car1_medium"
		texturefile = "gfx/interface/technologies/usa_armored_car_equipment_1.dds"
	}

The files can either be in the mod files like so:
<MOD_NAME>/gfx/interface/technologies/usa_armored_car_equipment_1.dds

Or in the main mod files like so:
Hearts of Iron IV/gfx/interface/technologies/usa_armored_car_equipment_1.dds

Or - and this is the one that isn't working - in the DLC folder like so:
Hearts of Iron IV/dlc/<DLC_NAME>/gfx/interface/technologies/usa_armored_car_equipment_1.dds