Assembling a car stereo can be an expensive process and usually involves building in stages. This, of course, means that at certain stages of building your sound system, your stereo will be incomplete, yet still in use (since obviously you will still need a stereo while you're in the process of installing the system). Thus, it is of course necessary to address the functional capacity of an especially powerful car stereo while it is still coupled with only a limited number of speakers.
Multichannel car stereos, such as a four-speaker stereo, work by evenly dividing the stereo's available wattage across a number of channels so that each speaker receives an equal amount of wattage. However, if two of the four channels in a four-channel car stereo receiver are left unused, the wattage from the stereo will be proportioned across the two channels that are in use.
Car speakers are designed to handle a set amount of wattage, which your stereo receiver runs through its channels. While cheap speakers are limited to around 50 to 75 watts, more expensive speakers are capable of handling more than twice that amount. If you are using the wattage provided from a four-channel stereo across only two speakers, there will be a higher than normal amount of power run through your speakers, so you should try to find speakers that are capable of handling the amount of wattage that you will be proportioning to your two available speakers.
There are many benefits of having a multichannel stereo even if you do not plan on using all of the stereo's channels. First, the amount of wattage that runs through the channels in use will be higher than with a single-channel stereo. Additionally, having a multichannel stereo gives you the option of adding speakers to your stereo in the future without having to replace your stereo receiver.
If you do plan on using the remaining channels of your four-channel stereo in the future, it would be best to run the wiring for the remaining two channels because the process would not be much more difficult since you will already be running the wires for the two speakers that are in use. Once you have run the wiring, be sure to tuck the wiring out of the way (under your car's mats and inside your door paneling) so that you won't be tripping on the loose wires.
When leaving two of the four channels in a four channel stereo unused, it is safest to cap off any loose wires that are left from the unused channels. Exposed wires are dangerous because, though the channels do not have speakers hooked up to them, there is still electricity running through the channels; this can shock you or even short-circuit the stereo. You can take precaution with your loose wires by cutting the ends of the loose wires so that the wire sheath covers any exposed wire, or using electrical tape to cover up any exposed wire.
