Hi,
Seems to me you are writing about two's issues. The alternator not charging and no dash warning lamps.

I suggest you look through this link and study the combined instrument section to see how the dash lights get their power. The fuses in the fuse panel get corroded or a ground wire on the cluster could be loose.
http://www.volvowiringdiagrams.com/volvo/240%20Wiring%20Diagrams/TP31054-1%201987%20240%20Wiring%20Diagrams.pdf

In position two, all the cluster lights go through a bulb light testing. Power not coming from fuses or the ignition switch to the cluster in or out to grounds can cause all to go dark.

The alternator exciter wire gets its power from the ignition switch and that battery light bulb that limits the current to the regulator.
That wire does run out through the gray connector tha Goatman is talking about. The bulb comes on when it see the ground path through the alternator's circuit containing the regulator module.

The lights goes out when the alternator starts putting out almost any voltage to oppose the current path from the bulb. The bulb is an "idiot light" as it does not indicate the amount of current or amount of voltage over the battery resting charge.
The alternator could put out 10 volts and the light might faintly flicker, if you are lucky, it gives you a heads up.
The best choice is to have a voltmeter in the dash and get use to see how it behaves under normal good operation. Loads of the blower fan, seat heaters order rear defroster can also be monitored.
The next choice is to be aware of how bright your headlights stay and that they do not flicker or stay dimmer at some time or another. Loose belts or brush issues can start that issue.

If you are looking, listening and smelling the car will talk to you.
Chewing bubble gum, well, that doesn't count until the ash tray is full! (:-)

Phil