It's all about the reverb. For "behind", the "direct" sound has to go through the auricle (giving it frequency-specific phase changes and attenuation) with the first reflection from the front being more neutral. For "frontal", the amplitude/phase relations are almost opposite.
This requires keeping tabs on several frequencies and their envelopes. Something like a continuous sine wave is much harder to locate: basically you do that by "ear saccades", moving your head and tracing how the sound reacts to that. Of course, that does not work with a stereo recording unless accompanied by visuals giving you a cue for the purported movement.
Without movement and without a somewhat reverbing environment, front/back location would not really work well. You'd have to rely on wide-spectrum sound sources with known phase relations (motor noise, click trains) and even that would be iffy.
The point being that usually a number of different cues individually adapted to auricle form are employed and combined, and most of them work to some higher or lesser degree even in isolation. But usually they are employed in a complex ensemble.