Wiring question

[GeorgeLeonard]

As this question will indicate, I dont know much about automotive wiring. I recently purchased a 1966 Chevy truck project and the wiring is a mess so I am sure to learn a few things while trying to figure it out. The guy I bought it from used to connect the battery terminals every time he used it and disconnect the terminals whenever he turned the engine off. He said that the battery would drain if he didnt do that. Whatever.

But he told me that when re connecting the positive terminal, to be sure that I also included a certain red wire which he showed me. Fine. I do exactly what he told me and everything is as it should be- starts and runs no problem. When I dont connect the red wire- it will not start. So Im curious and I start to look around and see the red wire is hooked up the the horn relay? So I look at the wiring diagram and see it is supposed to be just that way- this seemingly life giving red wire is connected to the horn relay. It just seems so counter intuitive that such an important wire would come from the horn relay. Can someone make some sense of this for me? As anyone who must be considering answering this question must already know, it goes from the horn relay to the voltage regulator and then to the alternator.

Why would this wire stop my engine from starting? What does this wire really do? Or are things more messed up than I think they are? The good news is that when the wire is hooked up to the battery, it starts and runs pretty good. Has it been hot wired or anything like that?