What's draining my battery?
1: Faulty Battery: inspect and fill it with distilled water, if needed: battery may be shorted out
2: Alternator: the voltage regulator inside alternator will drain battery when it quits working: break unit open, and replace the regulator: or replace the entire unit with a rebuilt alternator.
3: Loose alternator belt, which slips and does not allow for enough revolutions to fully charge the battery - especially at night-time, when you are running your headlights and such.
Now, to do a test, which will tell you if the alternator has failed, in 10 minutes:
1) Simply loosen, but Do Not Remove, the battery post clamps / cables, from battery.
2) Start Engine.
3) With engine running, REMOVE both cables from battery posts (engine should continue to run) (if it does not / dies out upon removal of cables, then test no further): ensure to secure the positive post cable - wrap it with a rag, or set it on cardboard or rubber, so it does not ground: then reach inside, and turn ON your headlights: upon doing so, and highlights are ignited, if the engine dies out - if the engine quits running the instant you turn your lights on, with the battery cable disconnected off - that means that the alternator is Not Charging; that it is not producing enough current to run itself / maintain itself running / maintain the engine running; so it does not produce enough electricity to keep your battery fully charged - and, is in fact, draining your battery of any available current which was stored in it.
So, more likely then not, it's the electronic regulator / diode stored inside of the large alternator unit: but, because, your car is a 1990, the brushes on your alternator are shot anyway; so you are better off just replacing the whole unit and belt: and battery if need be.
I hope this helps somewhat: where, with ENGINE RUNNING at normal / higher idle speed, and BATTERY DISCONNECTED, and ACCESSORIES RUNNING, especially headlights and heater - the Engine Must Remain ON / Running - on solely alternator power.
In that: the battery is only required to start the car; stored energy / current for the purpose of starting the car, as supplied by the alternator - and adjusted by the regulator: a faulty regulator will fry your battery, by over-charging / allowing too much electricity to flow.