What is happening to you Dan (I mean your car, actually) is NORMAL. The alternator puts out an anaemic amount of current to begin with. Then, the OEM headlight wiring is marginal for the job. If you have the original size lights (45 Watts at low, IIRC) they aren't too bright to begin with. (Kind of like certain Pace Academy Students). If you have replaced them with higher wattage bulbs, they are even more limited at idle. You can either live with the dimming (kind of a courtesy lighting at intersections), or turn up your idle speed a bit to compensate. Or, you can go to a heavier duty alternator, and you can upgrade the wiring harness, as far as the headlights go, to improve current flow. It is POSSIBLE that your brushes are getting worn and that's contributing to the dimming, but probably not. You can remove the alternator and the folks at NAPA will bench test it (probably for free). If it looks like it might be wearing out, then for about $7 you could buy new brushes. They are basically cubes of metal with wire pigtails that are held against the main alternator axle by springs. When they are new, things may even be temporarily worse, since their squared off surface has less contact than your old brushes, which have worn to shape, on the axle curve. As Bailey says, replacing the brushes is mainly a job of opening the housing with a screwdriver, detaching the pigtail with a soldering iron, and soldering on a new brush. (When I decided to do this, I found that the brushes were worn, but all I had to do was lengthen the pigtail and the spring pushed them further in and they had new life.) The toughest part is putting the alternator back together, because you have to push the brush into its spring and hold it there. The secret to that is the paper clip wire sized hole in the housing that you use to stick a wire through to hold the brush back.
Speaking of Netter, you would be very happy owning a set of OEM Manuals (Engine Manual and Chassis and Body Manual) for your rig. Makes life so much easier!