From Datsun 1200 Wiki

Datsun 1200 used dual horns. It also uses a three-connector relay, which is bolted to the right-side strut tower just below the ignition coil.

 (http://datsun1200.com/modules/myalbum/photo.php?lid=14969)
#3: horn relay

Wiring

Horn wiring schematic:  (http://datsun1200.com/modules/myalbum/photo.php?lid=14974)

Relay Wire Colors
 (http://datsun1200.com/modules/myalbum/photo.php?lid=14975)

• Green - to horns
• Green/white - to 15A fuse in fuse box, Battery side
• Black/green - to steering column connector (for horn button)
• Case bolts to body (earth/ground)

 (http://datsun1200.com/modules/myalbum/photo.php?lid=14970)
B120 Horn Relay

Stamped on the relay case are:

• H - connector for Horns
• B - connector for Battery (through fuse box)
• S - connector for Switch (in steering wheel)