What is this relay for?

[fly_25]

Hello everyone, does anyone know what this relay is for? The brown wire goes into the alternator and the rest of the wires go into the firewall. Any help is appreciated, thanks! It was mounted on the firewall next to the brake booster.

 

[fleming442]

Hit the black with a conventional test light. If the horn honks, it's the horn relay (just because green and black). If not, no idea