The horn needs to be removed. I'd suspect part of the problem is the bedding. Mounting anything on agressive, diamond patterned non-skid is going to be problematic.
I'd trace the outline of the moutning plate, then completely remove the horn. Then I would grind down the non-skid in the footprint of hte horn mount. I would sand, and then wet sand, then compound until the area was completely flat and smooth. Then I would use new SS bolts and nuts (screws if that is what was used), and some 3m 4000 (It is made for Deck hardware and is UV resistant), and rebod the horn again. I'd use the 3m around each of the screws as well as the entire perimeter of the base.