Aquarium heaters normally have the thermostat at the top by the cord/ cable and the heating coil is at the bottom end of the heater (further away from the cord).
Have the heater running in the tank and touch it with your fingers. See which part heats up. The bottom half should and that will be the heating coil. The area with the light should be cool and you can wrap that with a thin piece of black plastic. You only need the plastic to be slightly wider than the light, then just wrap the heater around the light.