Check your grounds.

On the RHS of the engine bay, you will find the horns. One is above the
windscreen wash tank and the other below. Check over the wiring there -
that's the most likely candidate.

Next, check the wiring behind each of the horn buttons. Do all of them fail
intermittently? Again, check the grounds, using a multimeter and placing one
probe against the ground of whatever you are checking and the other probe
connected to the chassis/shell. Using a continuity test, you should find
continuity (or no resistance if your meter does not have a continuity
function).

Happy hunting :)

Merry Christmas all,

Paul

1989 900 Turbo S
http://saab.go.dyndns.org/