You've got a unitised triplex switch in there: there is one spring-loaded switch behind each of the three soft plastic horn buttons . The switch completes the ground for the horn relay trigger (coil) circuit. Pry off the central 3-armed hard plastic steering wheel trim and find the wire. Ground it; if the horn sounds you need a new horn switch (which means finding a good used one or searching for NOS). If not then remove the steering wheel and clean the contact ring on the back of the wheel and the spring-loaded roller contact that sticks up from the turn signal switch. Ground that roller contact and the horn should sound. If not, the problem's in the wire between the turn signal switch and the horn relay.