First, are you sure this isn't "normal?" Are you saying that WITH THE ENGINE at a steady cruise speed, the lights were getting brighter and dimmer? Or was it just when you changed engine speed?

This problem can be caused by a number of things
First, this being winter, sometimes going through a puddle can throw engough water on the belts/ pulleys to cause things to slip for a short while.
CHECK THE BELT. This does not mean, "see how tight it is." Belts can get "slick" from slipping and (after being retightened), won't pull under load. Likewise, pulleys can wear, and if run for awhile with loose, slipping belts, the pulleys can get polished up so they won't hold well. New belts stretch, so if you replace a belt, you must run it for a day or two, and retighten.
Worn or sticky brushes. Pull out the brushes and instpect them, for free movement in the holders, and better yet, replace them It would be even better to remove the alternator, and inspect the slip rings. They should be smooth, if there is a lot of engine grease or road spray, that could be the problem. Of course they can be just plain worn out.
Loose stator windings. I've seen several Mopar alternators whereby the stator windings vibrated loose, and wore against each other until the insulation wore through. I'ts easy to tell--With the hood raised after dark, start the engine with an electrical load, like the heater turned on. "Goose" the throttle, while looking at the alternator, and look for sparking coming from the windings.
It can be a bad regulator, or one that is poorly grounded.

Now, just go through and check several connections.
Check the main battery cables, and clean them. The "main" batt ground should be to the block, and there should be at LEAST one jumper of #10 ga or larger from the block to the "frame." More is OK
Check the wire connections at the alternator, tight?
Check the firewall connector. If nothing else, pull it loose and "wipe" by working in and out several times.
CAREFULLLY reach up under the dash, feeling for heat with your fingers, and feel the ammeter wires, which are two "nut" connections. See if they are loose. With a heavy load on the electrical, as lights, heater, and the engine at least at a fast idle, wiggle the ammeter connections, and see if anything changes.
It could be a loose connection at the ignition switch, or a switch going bad. If you can disconnect the starter relay (on the firewall) after starting, watch the ammeter while wiggling the key/ switch.
Last, somewhere up under the dash, in the "main" harness near the ignition, is a great big splice, wrapped up in the harness. In the past, I've found at least 2 Mopars where this splice had come loose. I believe it originally was spot welded, and evidently can break from flexing, or maybe poor quality control.

 

Boy, "you" (or somebody) really knows how to through a wrench in the works!!

I keep forgetting, that as old as these cars are getting to be, almost ANYTHING can happen to them.

(Note to self: Check for wiring mistakes)