Okay, my dad just got home and said he had problems with the '89 240. I can't remember everything he said exactly, because it was a lot. But, the gist was:

-When he starts the car cold, the OD arrow light is lit. Hitting the button shuts it off.
-The speedo/odo stop working sometimes. Hitting the OD button gets it going, usually.
-The battery discharge light lights up brightly. Often in conjuction with an unwanted OD arrow lighting.
-The batterry discharge light is also starting to light on and off on it's own. Always bright and solid, no flicker. Goes off. Often by turning the OD on and off.

I recently replaced the negative battery cable. The car had been lightly flickering it's battery light, and the old blue battery cable looked pretty mangled, so I replaced it. I didn't use the OEM-style blue cable, just some SMP-brand ones. I checked and cleaned those connections and they're fine, but I'm just saying this because it means that the car was acting up electrically before. The flickering stopped, but apparently now we've moved on to worse things.

Other details:
-Battery is less than 8 months old.
-Car starts right up with no problem.
-OD works splendidly.
-Speedo and odo work fine most of the time, except for the times I mention above.

Right after my dad told me all of this, I took the car out for a drive. This is what happened there:
-Started car.
-OD arrow was on. Shut it off with the button. Dad did not leave the OD off, so the arrow should not have been lit.
-Started driving. Speedo wasn't working properly.
-Once on main road, I hit the OD button to test the OD unit. The battery discharge light lit up as bright red as possible for a couple seconds and the speedometer jolted to life.
-Everything was fine after that.

I'm going to let it sit overnight and try it tommorrow and see what I get, and give an update.

Any ideas? I was thinking maybe the voltage regulator or something, but electrical stuff is not my forte. Nothing else is acting up, and all the other electrical stuff is just fine.