The ears of all mammals enter the skull at its base, just behind the jaw. This is true of both humans, and fennec foxes. Fox ears on top of a human head are thus anatomically plausible without seriously changing the anatomy of the skull.
However, there are obviously other anatomical differences. Because a fox's ears are on top of its head, it has a very long ear canal that ends in an "L" shape before going into the temporal bone. This is why it's safe to use a Q-tip to clean deeply in your dog's ears but not your own. The human ear canal is a straight line, and you could poke your ear drum. Not so in canids.
But if you want the ears to behave in a similar manner to a fox (i.e. to move around and point at things), you're going to need a lot of skeletal musculature to move them. This means the addition of muscles on top of and behind the human skull in the occipital bun region (basically the back of the head). These muscles would almost certainly make your fox-humans' heads look larger than us regular ape types. The fox-humans' ear canals would also go along the side of their heads until opening into the ear lobes on top. This might add some extra "bulk" to the side of their head/face area as well.
Another question is where you will get the blood supply that you intend to cool through these ears. Human heads keep all their major blood vessels on the inside for good reason. In order for these ears to act as radiators, they will need a lot of blood supplied to them, which would make them a prime target for attack, and any damage would likely result in massive blood loss. But the real question is how to get them that blood in the first place. In normal humans, the scalp is provided blood by three arteries: The superficial temporal artery, the posterior aricular artery, and the occipital artery. The largest of these is the superficial temporal artery, and luckily enough it goes right behind the jaw just in front of the ear canal, a perfect position for your fox-human ears. You'll need to make this artery much larger, which will probably put some pressure on the neck area and just behind the jaw, plus you'll need a new branch off of it to feed into the fox-ears (as the normal temporal artery is already busy branching off into smaller tributaries to feed the scalp).
In order to be effective radiators, the ears would have to sweat... A lot... Which would likely make them wet all the time and quite possibly cold to the touch. If the ears were ever dry, they'd have the opposite of your intended effect, as they would absorb heat from the surroundings and channel it into the blood. You'll probably want your fox-humans' anatomies to restrict blood flow to and from the ears based on body temperature. This would happen at the autonomic level.
In terms of what these ears change as far as abilities go, well the answer is going to be not a whole lot. They'll definitely make your fox-humans better able to hear, but only by maybe 10 dB overall. They'll be unlikely to hear all that much better than a regular human. But the fact that the ears act as collectors will markedly change the directionality of their hearing, and their hearing would likely be worse compared to a normal human behind them and away from the direction their ears are pointed. Normal humans hear roughly the same in all directions, as our ear lobes don't really do anything at all to change the directionality of our hearing. Your fox-humans will however be able to tell what direction a sound is coming from much better than a normal human, as they can tell by trial and error by pointing their ears in different directions.
Your fox-humans also won't hear any more of the acoustic spectrum than a regular human, since all of that is determined by the size and shape of the cochlea, as well as the number of hairs present and the carrying capability of the acoustic nerve, all of which would probably remain unchanged.