Do you know what happened to the original fuse? Well, the best advice I can give is you don't want to put the wrong amp fuse in. The fuse is a safety device that prevents the wires from burning up and messing up whatever the circuit is feeding. So in a nutshell it is the weakest link of the circuit. Is there any indication on the wire (18awg/20awg)? I doubt there will be any markings on the wire, but this is what the size amp fused is based on. Your best bet as cletis stated you might want to trace the wire to see where it goes.
