I'm taking it you have the "older" model smart? I actually accessed through the driver's side wheel well and installed a wolo bad boy air horn in my '8 and '09 passions, want to do the same to my 2 '16 passions but I haven't found the horns yet.
If you want to add a second horn to add to the stock horn,it would be best to put them on either side of the front. Best way is to remove the front clip and then you are able to run wires more easily. Trying to work through a wheel well, or by flexing a fender can cause damage.
I pulled the front end and switched to dual louder Wolo electric horns. It sounded like a big Chevy when I was done but other cars got out of the way.