Electrical problem

[zac]

1979 K20 Camper Special. Had lots of PO wiring issues, think I have corrected them. Has new battery cables, new alternator harness, inspected headlight harness and see no issues. Tried 3 different alternators with same result. Driving with headlights on, brightness is very erratic, can help things by goosing the accelerator. But issue doesn't seem to be RPM related. Sitting at idle it's fine many times, except when it's not. Driving at 65-70 you notice lights start to dim, so you play with the pedal and you get a temporary fix. When lights are dim, voltmeter shows low, just above red arc. When they're bright, it shows 12-14. Other accessories make voltmeter dip as well, but seems more normal. The headlight issue is clearly abnormal. Running out of ideas. Could this just be the light switch?