Geez, great group of geniuses here. A radio does draw some energy away from the engine, as some people have indicated, the alternator is regulated, and draws considerable power when needed. However, the biggest gas guzzler in your car isn’t the radio, the defroster, or anything like that.
Very simply, any electrical device that converts energy to motion will use considerably more energy. While bouncing a few electrons around a radio, or defroster does take some effort, it’s nothing compared to the effort needed to turn even the smallest fan. In essence, if you want to save gas money, turn off the heater/air conditioner fan. BTW, whether you have the heat on or not doesn’t matter, as the heat itself is essentially a byproduct of running your engine.
If you have any doubts, there is a very easy way to check this. Drive with your fan on full blast. Then with your heater on hot, with the fan on high speed. Then your heater on, but the fan on low. The same for A/C, in the summer. You won’t even need to run through a quarter tank of gas before you notice the difference. With the fan on high, you will burn much more gas in the same driving situation, as your typical car only draws the same energy for the engine to run as the headlights do, but less than the fan does on high. Unhook the alternator sometime, and test out different scenarios with a fully charged battery, and you’ll see real quick what that stupid little fan can do to your power usage. 