I just wrote a long post, but my browser lost it - internet cut out on me.

Anyway, the switch is probably not your problem. I took apart mine (90 740T) and it's a Y-switch that connects power to one circuit (park lights) or two circuits (park lights and headlight relay). The headlights are on a relay system to prevent excessive heat/current being run to a switch and to cut down on wiring (and reduce heat produced through resistance).

It sounds to me like you are somehow feeding the headlight circuit directly with current from the parking lights. I don't really know how it's happening, but a crosswired or otherwise miswired relay could be the problem. I more suspect the wiring has somehow rubbed through insulation and is cross-connecting the two circuits.

My first diagnostic would be to see what happens when you pull the fuses for the right and left side parking lights. uh.. numbers 21 and 22 I think. If the headlights are staying on, then there is more likely a stuck relay or other wiring problem and if they go out... well you get to find a short of some sort.
So you're not surprised, the headlights should always work if you pull the turn signal stalk towards you. It's a flash to pass system (on when you pull the stalk). The hi-beams are on some sort of separate circuit I don't understand, but I think it is built into the headlight relay.

Good Luck
-Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond