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1978 r100s having electrical problems. headlamp low-beam would power off randomly, seeming to be a short in the head light can. now it doesn't power on, just head running light and high beam, and seems to be on the same circuit as the tail light because the started failing together. sometimes the tail running light works and the brake levers both brighten the light, other times it just stays bright without engaging the leavers , sometimes the levers brighten the tail but I'll have no running light. when the headlamp was randomly turning on and off, if the headlamp was on--there was no problems with the tail. fuses are good, and i checked all the connections inside the can(it's pretty clean in there)and under the tank(a little messy but everything has contact. I fell like I'm chasing a phantom electrical problem.
ahhh! what do i do? electrical is the devil

Do your symptoms seem to happen when you are turning the bars. Almost sounds like there is a problem inside a harness when it is getting flexed during turning. Possibly a coup.le of wires where the insulation has worn through, touching when the harness is flexed. I would pull the tank, turn on the lighting circuit and squeeze and work the harness to see what areas it may be sensitive

I had occasional problems with my fuses - had to give them a twist in their holders to maintain contact, until I replaced them with new blade-type fuse holders. The other thing is that your running and tail lights are on a different circuit to the brake light, but use the same earth path. I'd be checking all the brown wires for continuity (a cheap multimeter with continuity beep function will help). There is a major earth connection to the frame near the coil mount, and one of those brown wires runs back to the tail unit. More brown wires terminate on the circuit board in the headlight bucket.

Do you have a circuit diagram? If not, send me an email or PM (include your email address) and I'll scan the colour diagram in my Haynes manual and send by return email. Demons can be beaten by methodical investigation.