Well it sounds like you got it sorted so I’m not sure how much help, if any, this will be. The Blue/White is your 12V active horn wire.
Just for the sake of how to best use the multimeter with this kind of situation. Here goes:
On your type of meter, the left side is for measuring DC voltages in different display ranges. On the right of that are two AC voltage ranges. Towards the bottom there is a setting for the continuity test (more on that below).
So, to find the active horn wire, disconnect the leads or plug from your horn, then press the steering wheel button. Set the meter to the “20” position and then put the red probe on the wire you suspect of being the active (e.g. blue/white in this case). Put the negative probe to ground (against the chassis or the negative battery terminal). The meter should now read positive 12V (make sure it’s not showing negative 12V). You can swap the probes around for the fun of it and repeat the test to see -12V on the display (‘yay’!).
There you go. Done!
Now just something on the continuity test. It basically uses an internal 9V battery in the meter to send a low voltage current through the probes (and whatever sits in between them). The meter will beep if you have a conductive path (i.e. allowing continuous flow of electrons to pass between the probe points; You can test it by holding the probes together). It also shows you the resistance on the display to indicate how conductive the path is, but I won’t go into that here.
The continuity check is great for finding earth/ground wires quickly for circuits that aren’t live. So without pressing any horn buttons, put the negative probe against the metal of the chassis somewhere and then probe the wire. When you get the beep tone… bingo - you get the drift. You can confirm your find by switching the meter to the DC mode to ensure you haven’t tapped into a live wire.
