What Jon said abive. Some of the bullet connectors around the wing stays need checking. I lost a light on the offside a while ago and that was why.
The horns earth at the steering wheel boss, so check the spade connector is still connected.
The S1 Roadster has two horns.check the live feed to each, and test the earth return. They are under the front panel.
The fuse is very marginal for twin horns, and I upgraded mine to....... from memory 15A.
It's possible you have a hairline crack in the fuse, which will allow the radio to work, but overheat with horns, so try changing it.
