Headlamp shorts rear light! Confused.
 

When my headlight is on the rear tail light and the speedo lights go out. If the headlight is off but I've got the parking lights on the rear light and instrument bulbs all work fine!

Presumably some sort of short. Anyone have any ideas as to where to look or how I could fix?

Which bike/model?

Sorry -:(
Forgotten what caused it on my R80/7 1981.
I think I have had it at two places though; handlbar switch and inside the headlamp (fuse gone, current going around?...), and I think I had a relay problems once to (inside the headlamp that to). But I am very far from sure - simply forgotten it.

Its an r100gs. I've had the multimeter out and discovered that the problem stems from the ignition switch. Its recently been worked on and obvioulsy not put back together very well. I've had to buy a new one. Never mind.

when I had to change the ignition on my 1990 GS, I had similar problems. Check the connections onto the ignition switch, I had to experiment with the connections because I didn't pay enough attention when taking the old switch out.