"91 Legacy Electrical Problems...

Hey, I have been reading here for quite awhile.. :) Pretty nice site going here, can usually find everything I need. However I am having some problems with my 1991 Legacy AWD Sedan. For some reason when I turn the headlights or brakelights on everything is pretty dim. The brakelights are fine, but the headlights are always dim when on, even on brights. Lights inside the car also get pretty low when I turn the headlights on or push on the brakes. The radio light goes away all together, and can hardly see the dash.
Kind of at a loss, I think it is the alternator maybe? not sure. Always nice to see at night :P

I usually like to find out whats wrong and fix it before I give up and take it to a mechanic :)