This is from mounting instructions for PIaa horns. Note number 3:

1. Do not remove or alter the screw or the bracket on the backside of the horns. Doing so may alter their performance or may disable them.
2. Install the horns so the open end with the cover points down.
3. Do not install and wire the PIAA car horns so they work with the factory installed horns. Doing so may overload the circuit or may not pass local noise regulations.

I guess if you start blowing horn fuses you will have to disconnect the stock horns.