'65 starting troubleshoot

[Dappy]

I am looking for some troubleshooting ideas please.... Electrical is not my strength. A few weeks ago I bought an old beater. 65 body on a 74 frame, 3/4 ton FWD with a newly rebuilt 350. From the firewall forward, all the wiring is new, within 5 years or so, and does not match any wiring diagrams. The guy I bought it from said it drains the battery and he has installed a quick disconnect on the battery, so it must be disconnected and reconnected every time the truck is driven. I put a new voltage meter on the dash, and it registers 13 volts at idle, and 14 volts going down the road, and 13 volts with headlights on. With the truck stopped, when I disconnect the battery there is a heavy click (one) that comes from the horn relay. I just replaced the voltage regulator and the horn relay with new ones and the click remains. With the + cable disconnected, of course, there is no drain on the battery. The moment I connect the + cable however, the single click happens at the horn relay, and the battery discharges itself pretty quickly unless I start the truck. Starting the truck brings good voltage. I had the alternator tested at the shop, in the truck and truck running, and it is charging fine. Also... according to the wiring diagrams, the + lead on the battery should be connected directly to the horn relay.... but it is not. Also, the third wire down on the 4 wire clip attached to the voltage regulator has been cut. It looks as though that was done 20 years ago... the wiring diagram shows it going to the horn relay as well. Anyway... I would sure appreciate some ideas. The truck runs like a champ and charges fine and produces loads of electricity. I just want to get rid of this hassle of connecting/disconnecting the battery every time I drive it.