I would find a '70 service manual with the electrical schematic which should show which plug and pin on the firewall bulkhead connector is for the horn and then trace the engine harness wire to find out where it terminates. I only have a '73 manual. On the '73 it is a dark green wire on the "K" pin. The oil pressure sending unit, regulator and ballast wires are on the same plug. IIRC one plug is for front lights, one for engine and one for accessories.