To function test the switches, can I attach the red lead to positive on a charged 12v battery, my multimeter negative to the battery negative, then look for voltage at the various wire ends in all the switch positions? Or will I blow something out at the switch? Could I use a household 9v or something instead of a vehicle battery?Also, how does the horn ground?? I get continuity from the purple input wire to the inside of the horn button when I depress the button, and it cuts off when I release it. Is the switch just grounded by being in contact with the handlebars? That seems like a sketchy connection, especially since my bars are powdercoated.
For future reference, you can use any voltage. Just keep it below 12v as the switch will have a max rating. However, you don't even need to do this. Simply switch your multimeter to resistance mode and see when you close the circuit. No voltage needed.Regarding the horn. Many cars/bikes have a common ground. The negative terminal of the battery is usually wired directly to the frame and/or engine. As you surmised, paint, dirt, etc. can cause issues with this type of setup. Saves a lot of wiring though! Poke around till you see where it should be contacting the bare frame and sand a clean spot there. I've noticed most things on my RE have redundant ground wires. I've actually removed them on the turn signals and brake light.
The horn problem is that there simply is no ground wire coming off the switch for the horn.