from what you're describing, it sounds like there was some janky wiring done in there with bridging circuits or tapping into circuits one shouldn't use for power. if the headlight circuit is being used for those other devices, i believe it's likely someone got in there and crossed some wires. i suppose it's remotely possible that there could be something honked up in your fusebox, but i think that's far-fetched.

to your knowledge has there ever been any work done in there? i'm thinking aftermarket stereo for instance, where someone's in search of +12v and whatever they find, they vampire-tap into and just go with it. i know this because that's what i did in my truck when i first was messing about. i've since rewired in an entirely different way using an auxiliary battery.

so, i would definitely be looking very hard at any custom or loose wiring in the dash, and attempting to determine if it's vampired to any leads it shouldn't be. i would also just have a quick look in the fusebox (the one under the hood) just to verify that no obvious cross-circuit bridging is there.