Headlight Issue

[Zilverado]

So, it seems like when it rains it pours... lots of little nit-picky things starting to crop up all of a sudden...

My '86 Chevy has the 4-lamp headlight system. I noticed tonight on my way home that I was missing the low beam on the passenger side. Thought to myself - "no biggie, just pick up a new lamp tomorrow and away we go". Well, it turns out that the lamp does come on when I turn on the high beams. Then all 4 lamps are on.

I just got home and it's late and dark and I'm tired, so I didn't even start to check things like wires and connections, but I thought I would quickly ask for some thoughts.

So what gives here? Could this be a headlight switch thing (please say no ? What else???