Are these by some chance on the same circuit at the fuse box? If it's a fuse, is this all that won't work, or did I miss something else that should be not working?
The fuel gauge failed by showing FULL, when in fact it's almost completely empty. Horn just won't honk.

 

Separate circuits. The horn goes through the C100 into the dash harness (green) and then to the relay at the convenience center. If you take the center of the steering wheel off you can see the spring contact for the horn. Its fragile, perhaps it disengaged. I would also check the relay (actually, do this first).

As for fuel, that circuit runs along the driver side door sill and through a bulkhead in between the rear seats. The wire for the gage is purple. If your fuel sending unit is original, it may be stuck and or just plain dead.

 

As it was said two different circuits