In the case you are boosting your Mustang’s sound output, you are going to increase the amount of power the system is going to need to operate. For basic upgrades you might not have to be too worried about the factory electrical system, but you need to understand what you are doing to make sure you don’t push it too far.
The factory battery may work well enough, but it’s not bad idea to upgrade to a beefier unit as these upgrades pile up. The battery’s size doesn’t really matter without the proper alternator.
The alternator’s job is to restore the battery as power is drawn from it. The more you draw from it the more crucial the alternator’s job becomes. As you modify the car’s stereo system, you are going to pull more and more from the battery and the factory alternator may not cut it.
If you are going to make extreme modifications you have to beef up your alternator up, otherwise you will not be able to power it. Last but not least, you need to ensure you are using the proper gauge wire when building the stereo system.
For light upgrades thinner wires are ok, but when electrical output increases, thicker wires will be needed to handle it. If you push too much through your wires you can wind up burning your car to the ground. So please, for your own safety, be sure to use the proper wiring. Rock and roll may not preach safety but without it, rock and roll wouldn’t be rock and roll.