That is the way the early AD trucks were wired and that was on six volts. A twelve volt horn should not take as much current. Now, on the other hand I don't know if your horn contact set is as substantial as the one in the AD truck

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!