On off switch???..........i that the kill switch or the Ignition switch.???
Test the red wire form the battery, +, to the battery side of the Fuse box.
Test the red wire from the battery, +, to the far side, Ignition side of the Fuse box. through the main, (20amp), fuse.
If no voltage drop, test the red wire from the fuse box, (main fuse), to the ignition switch.
Now test the red wire from the fuse box to the brown wire after the ignition switch......This will test for a drop in voltage through the ignition switch.
I didn't understand your explanation.
The kill switch can also b a cause of a potential voltage drop. May need to be dismantled and cleaned.
The bike has been sitting for decades. The whole loom connectors, both plastic and bullet, need to be dismantled cleaned, checked for corrosion and any damage repaired replaced and reassembled, using a conductive grease. All earth/grounds removed cleaned and resembled.......All switches, Ignition, Kill, left and right bar switches need to be taken apart checked for loose or broken wires, corrosion on contacts, cleaned and reassembled.
I done my loom and switches doing a continuity test on every wire between connectors then after a connector then between that connector and the next on every circuit. Tedious but after going through the whole loom without any continuity/resistance i picked up any voltage drop, when i did run live power there was next to no loss, any more than would normally b expected after multiple connections.
I put a second hand loom on my bike, some before and after shots of the bullet connectors.