[particlewave]

I'm guessing that they are halogen?
If so, there is a small bulb that operates as a running light and is on with the headlights and/or running lights. I think your headlight did burn out and what you are seeing is the running light because it is behind the lens with the low/high beam bulbs and looks exactly like a headlamp is going dim, like you described.

Turn your headlight switch to running light position and verify this.