2006 Mustang V6 subwoofer barley getting power.
I recently installed an aftermarket sub woofer into my 2006 v6 mustang and at first everything was great. great sound, even tone. then about a day later i couldn't hear it when the music was fairly loud. i reved the engine pretty loud (hitting 3-4k) and low and behold i hear the subs. then about after 4 -5 seconds of playing after i reved it the sound fades and i can no longer hear them. now my battery light has been on for a month or so prior to the sub install. how can i tell what is the problem (bad battery, alternator is shot, ect...) and how do i go about fixing it?