that does sound like the key switch. Same debug though, check with a meter from the battery - if you have 12V on the battery side of the switch but nothing on the other side when its on, thats where its getting lost. When you turn the key on make sure the battery side of the switch still has 12V. With a really weak battery even the load of a headlight can pull it down to 0V.
Also, a corroded splice or fuse might conduct 12V thru when it has no load, but it wont conduct the 4 Amps needed by the headlight, and the power at the switch input would goto zero volts when you turn the switch on.
the simple way is to turn the key on, start at the battery and work forwards.
Last edited by KCW; 02-22-2018 at 12:35 PM.