there is a 12 gauge wire that is connected the right post on the horn relay, this wire runs along the firewall, then runs through an in-line fuse to a plug.
Anyone know what this is? It's a 69 with Ac.
http://i7.photobucket.com/albums/y265/FloridaRodder/HornRelay.jpg

Thats the a/c blower motor wire that runs thru the wire gutter behind the engine on the firewall and ends at the suitcase box in a relay to control high speed fan when your a/c is on 'high' setting.
Pretty sure thats what you have going there. :)