Mopar did a "bad thing" with wiring in these, especially now that folks want to run stereos, pumps, fans, etc. ALL the high power conductors for the vehicle go through the bulkhead connector, sometimes two or three times, IE Alternator feed comes IN to ammeter, back OUT to battery, then headlights go OUT to lights, etc.

Also it has been alleged that the internal design of the older alternators (round back) are such that they simply do not "put out" much at low RPM.

I cannot personally confirm this, it may be true. You CAN use a newer "squareback" 70/ later alternator on your 69, by grounding the second field terminal.

So a combination of upgrading the alternator and look up the "Mad electrical" articles on ammeters and bulkhead connectors.

If you aren't a "concourse" freak, there are some ways to upgrade the wire size and keep the ammeter. One way is to drill out the bulkhead connector and slip larger wires right through with no connectors on them.

Also, on your dash lights::

Clean grounds, clean the sockets, clean or replace the bulbs. Make sure the bulbs are the recommended part no. There are MANY different bulb wattages of the same bulb size, so if you get a "heavy" bulb in one hole and a "light" bulb in another, you'll have a big difference in brightness.

Dirty, corroded instrument lamp dimmers don't help, either. You can actually reach a situation where as the charging voltage drops, the rusty wiper on the dimmer can start to arc and lose connection. Most of the time you can just twist the dimmer back and forth vigorously to clean up the slider.