Yeah thats right. most horn buttons switch a ground signal or -ve signal to a relay, which then switches the +ve feed to the horn, it gets its -ve side from the chassis. its the +12v feed to the horn that is the line with a fuse in it.
and yes the best way to check if its a short on in the wire to the horn is to, disconnect the horn, and do a continuity test between the +ve feed to the horn (on a relay usually pin 87 on a bosch relay), and a chassis ground. if you get continuity its the wiring to the horn which will be mainly in th engine bay.
edit: just went and checked the relay and the horn relay pin to test is pin number 3, nissan number thiers differently.