. Perform the tests in the link below:
http://www.askmehelpdesk.com/cars-tr...ml#post1973613
It sounds like the voltage regulator is bad, which is easy to replace on Fords, or the stereo is stressing the battery/charging system too much. Consider installing a larger battery, with greater amperage, and upgrading the alternator, if the problem persists. The stress may be too much and the battery and alternator are being destroyed. Talk to a stereo installer and see what they advise.
. Install battery cover, to prevent the battery from being damaged by under hood heat--very important.
Many auto parts stores will test alternators and batteries for free.