A lower watt ceramic heat emitter will help during the winter. The heat the air in the enclosure and do not produce any light that will disturb their day/night cycle. I use these in my bearded dragon cages during the winter. Here is a link to one..
They can be used in a regular clamp lamp fixture and you can place them whatever distance from your cage to achieve the temperature that you need in your enclosure. Chameleons definitely do benefit from a temperature drop at night so be sure to take plenty of temperature readings to assure that everything is within the acceptable range. These can also be used during the day in the winter along with your normal basking lamp to ensure proper temperatures in colder enviroments. Just be sure that you are taking multiple temperature readings to keep everything in check