The ear doesn't actually find the source, it's how the sound is perceived across both ears.
A sound coming from the side is the most obvious case where you get blasted on 1 ear and a relative muffled identical sound on the other ear.
A sound coming from the front is balanced across both ears. Same as behind, except that the forward-biased shape of the ears will distort the acoustics, leaving a distinct distortion signature that tells you the sound is coming from behind you. (For reference, modern 3D video game engines mimic this distortion so you can tell when something is happening behind your character or the camera if you are wearing headphones).
Sound from above or below you will have slightly different distortion signatures due to how the shape of the ears is different from above versus below.