Check the yellow/white wire with the meter probe.  With the ignition switch on, the meter should read 12V.

If you have 12V at the black/red wire at the horn it should verify that the horn and the lights are getting 12V.  They all get their juice through this black/red wire circuit.

The horn is always hot.  As long as the ignition switch is on and the starter button is not pressed, the black/red wire circuit should be hot.  The horn button works by grounding the other horn terminal so juice can flow from the red/black wire, through the horn, through the black/blue wire to the horn button contacts and back to ground via the black/white wire.

So, the horn and the lights are powered up by the black/red wires.  They also share the same ground.  You can verify that the horn and lights have a good ground by checking continuity between the horn black/blue wire and a good ground.

Turn the ignition switch off.  Set the meter to ohms.  Connect one meter probe to the horn black/blue wire.  Connect the other meter lead to a good ground.  Then press the horn button and verify there is continuity between the black/blue wire and ground when the horn button is pressed.

So, one meter lead to the horn female connector on the black/blue wire.    

Press the horn button and the meter should read zero ohms or very, very close to zero ohms.  If the meter shows no continuity or high resistance, inspect and clean the connector pins in the left-hand multi-connector.  That would be black/white wire to black/white wire.  If those pins check out, then the horn button isn't making good contact, but that wouldn't prevent the lights from working.

If the power circuit checks out and the ground circuit checks out, the lights and horn should work.

Lots of good details and info, Mike!

Wiggling the positive battery cable got it going tells me bad battery connection, OR it worked because the handlebars were inadvertently moved/repositioned when the cable got wiggled.  To Verslagen’s point, the smallest detail could provide a clue.  You might not even notice to repositioned the handlebars.

Then, more things happened after the handlebars were swapped out.  My bet right now is that the contacts in the starter button switch are dirty (your first response).  The wiggling of the positive cable was unrelated and coincidental and it was movement of the handlebars.

I know when I was swapping out the throttle cable on my bike, that entire housing can get pretty dirty in there.  I think I used a 1/4 can of contact cleaner in there.

I hate electrical gremlins!  Continuing to follow this post and learn myself.

So, I did another vid that does the test of the circuit from the main ignition switch through the multi-connectors and handlebar switches.  The new test leaves the battery negative cable hooked up.  I found that if you disconnect the negative battery cable your test device starts to serve as the negative buss.  It doesn't work right.

I think you will find this vid easy to follow and it's a pretty fast check.  Let us know what you find.  Again, I apologize for any confusion.  I'm sure there are probably a few EEs, ETs and electricians on this forum.  Maybe one of them can comment on the energized negative bus (black/white) phenomenon.

https://www.youtube.com/watch?v=P1jqMqHoXxQ

I guess you could get a false positive test if the contacts are intermittent.  I personally would press the button repeatedly and with varying degrees of force, angle of attack, etc.  I guess I failed to mention that.  Thanks for calling it out.

I’ll let DBM explain, but you should have 12v+ with the ignition on BEFORE you touch the starter button.  And your lights should be ON when the starter button is NOT being pressed!  Once you press the starter button, the voltage in the yellow wire should drop down close to ZERO and the lights should go OFF.

This turning OFF the lights feature when the starter button is pressed is to allow as much battery power as possible to go to the starter relay and starter.

If you are getting no voltage with the ignition on and you are not touching the starter button, you have a power interruption in the line before it reaches the connection where you are testing.  It still sounds like your right side switch where the power button and kill switch live is the problem to me.