I've lost my left high beam on my US 91 240. Years ago, I had much the same problem, and my late mechanic almost immediately took care of it by cleaning a connector that was behind the battery, which I should have paid more attention to, but as I remember, he said that its proximity to the battery could cause the contacts in the plug to get corroded.
I found one round connector, which is the only one in the area. Is that the one? I figure to just hit all the plugs on the connector with contact cleaner, then brush them out with the skinny little round bronze brushes I grabbed at my local hardware store.
BTW, thanks for the link to cleanframetrap. I R&R'ed my broken door handle with no problem, and while I was at it, installed new door speakers in both doors. One of the probs, as we all know with 240 door speakers, is a lack of depth. I had tried 5.25" speakers with 1/2" depth extenders, but they were just too heavy for the flimsy door panels. This time I replaced the 525s with 4", and used American International SG-400-DX 3/4" Depth Extender for 4" Speakers. Muy bien!