My CSVT had a connector and mounting holes just waiting for the second horn.
I purchased the Freeway Blasters and they are very loud.
They report them to be like 130db of sound.
The car came with a horn on the driver side only but the second horn mounts on the passenger side with very little modifications.
any 2 horn set should work.
One will be a "High" note while the other will be a "Low" note.
Good luck, it makes a big difference in sound.
Gary
