Its normally the connections at the horn that have rusted away rather than the fuse, its the most forward thing on the vehicle and gets all the spray over it! However the fuse is a 10 Amp (Red);
if you remove the front grill and drivers side headlight you will see the wire from the loom (low down behind the under headlight panel) this is the positive supply, check its live when the horn is pressed, then there is a short earth wire to the body.
if twin horns are fitted the positive then jumps across the top of the rad to the passy side horn.