Question about electronics

[E.D.75GMC]

First off, I know almost nothing about electronics. That being said, when I'm driving my truck after dark and I slow and then stop at a red light my headlights and dash lights dim a very noticeable amount. The battery currently in the truck is good, and I believe the alternator is basically the same as OEM (I don't know its age though). The dimming has got much worse over the past few days and if I'm stopped in a turn lane the signal lights slow to the point where I can count two full seconds between the blinks. Is this my alternator dying? Or possibly something else?