Do not take any power to the horn push. It simply completes a ground return to the live horn when ignition switch in accessory or run position. This is a unique feed from ignition switch on a light green/white cable running to 10 amp fuse 9 and on to both horn and switched radio feed. If you don't have a radio you may not have noticed this feed has failed. There is no relay involved ex factory. With ignition switch in either of above positions check for presence of +12 volts with multimeter on light green/white cable at ignition switch, each side of fuse 9 then at horn itself all on same colour cable. Note the wiring loom caters for two horns, although I believe only one fitted on Plus 4. Do not reduce fuse 9 to 5 amp!
The horn push switched ground return runs back from horn on purple/black cable to horn push slip ring. No harm in dabbing a grounded test lead on this horn contact to check function with ignition switched accordingly. A very thin smear of dielectric grease on horn push slip ring will do no harm if you trace fault back here although you mention you have already checked.
