I just had to replace my battery because of those infernal rear reading lights being left on by my kids.
As soon as I hooked up the Negative terminal (last of course) the horn sounded continuously (boy is that loud). There is nothing in the manual about this, and I searched the forum about this after disconnecting it. I thought I had the wrong battery or something.
Turns out that I had to hook it up, horn blaring and all, then go inside and turn the key and shut off the A/C and everything that was commanded on by my wife. Ahhh, silence.
I am thinking about a Priority Start system, because my wife was peeved that she could not take the kids to school in the A.M. (the little angels).
Oh well, live and learn thought this might help someone with the blaring Horn sound when you replace the Battery.