>1) How much current does this 60 year old horn ACTUALLY draw?, not what was it specified to draw.
If it makes the same loud noise as a new horn, it draws the same Amps. Energy in, energy out. No magic.
>2) Even if I did workmanlike job on the connections, how much current is ACTUALLY flowing through those wires, ON MY BIKE, given the resistance in the various joints, sockets, and bullets?
If I "did workmanlike job on the connections", Alan's meter (or any other) switched to Ohms will show two or three tenths of an Ohm. That won't affect the number of Amps.
>You can turn it all on, turn on the blinkers, continuously blow the horn, everything, and watch the ammeter and see how much current that is, then add whatever safety factor YOU like and put that fuse in it ... not use the "specifications" and a standard safety factor.
Possibly US electrical component makers do it differently. Here, if a component's consumption is X Watts, that's what it is; Volts known, the Amps can be calculated. No guesswork.
>Not a huge deal, but just the right way to do it if you can.
Only measuring Amps, how will you know when you've made a mistake? You need to do the calculation first then either have confidence you can do a "workmanlike job" or measure to confirm.