I think you missed the point.
Unless something is wrong with the A/F gauge adding a 5 amp fuse won't fix your problem.
What size fuse do you have supplying power to that switch? If you had enough draw to burn an 18 gauge wire it's still going to be enough power to make a 14 gauge wire hot. You don't want hot.
You shouldn't have more than 15 amps running through a driving light circuit. If that's the same circuit as your headlamps imagine how quickly that line is going to smoke when you add that much more current.
Put the exterior lights on a relay. It will take a bunch of load off that switch and wire.