Horn does sometimes works, sometimes not

[fixkick]

it is a fact all old cars do this.
#1 is bad relay, and rust.
rust in horn mount or any of those grounds seen above.
and any connection end to end, there.
that is normal for old cars, and using meter is the only cure , if new relay fails.
try a new relay first, as most folks do.
relays do in fact go intermittent, when old. and there contacts wear and burn.