Got to fix the horn for the mot, thats the only thing it failed on

Whats the best way to go about it?
Ive checked the fuse and that is fine.
Should I then be able to check the actual horn is working by attaching a live straight from the battery to the terminal? Its a one wire one so is earthed by the mounting.
Ive tried both sides and neither work like this!
Can I put a test lamp from the live feed that connects to the horn and then put that to earth? Surely then the test lamp should come on when the horn is pressed?

Help!