What does this electrical box do?

[bmxmon]

After along time of ignoring my mustangs electrical problem, i've decided to finally deal with it. My problem is with this box i believe. When I start my mustang, the volts will run low (around 11) and my "GEN" light will stay on until I rev my engine up to around 25-3k, then the volts will jump to about 13-14, the GEN light will turn off and it will be fine at any rpm. When i then turn off my car, after the engine stops my GEN light will come on and stay on, until i disconnect the battery. (Though on occassion it will come back on after i reconnect it, if done right away) I believe it is this box that has gone haywire, since it clicks when i disconnect the battery. It just recently started doing the low volts thing on startup, requiring me to rev it. (Which makes me think my alternator is going bad also) Thanks for any info, I hate electrical systems....