...and I ran down to the local Kragen to pick up one of these horns:
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=WLO&MfrPartNumber=3852T&CategoryCode=3074
I only bought 1, it seems plenty loud. I used the original mount too, might be able to place a second on the mount too if one is not loud enough.
I might get one for the motorcycle too.
I suspect my horn was stuck "on" at the steering wheel (fixed that) but silent, and chewing up the battery.