Body heat is lost across surfaces, and small desert mammals face a hard trade-off between the surface area needed to shed heat and the water cost of doing it by evaporation. The fennec's ears solve part of that problem passively. They are broad, thin and richly vascularised, and blood routed through them radiates heat to the surroundings whenever the air is cooler than the animal.
The rest of the adaptation is behavioural and renal. Fennecs are nocturnal and spend the day in burrows dug into sand, where temperature and humidity are far more forgiving than at the surface. They concentrate urine efficiently and can subsist largely on moisture in food, and their footpads are furred, which insulates against hot sand and improves grip on loose dunes.
A common exaggeration is that fennecs never drink. They will drink when water is available; the accurate statement is that they can go long periods without it, which is a different claim and the one the evidence supports.