Don't exclude the possibility of an internally shorted bulb.
Check the wiring diagram and identify everything, and I mean EVERYthing, that is connected to that circuit. Chances are, this will be: Taillight (including brake lamp), license plate lamp, turn signal circuit, instrument lighting, horn, possibly running lights if so equipped. There may be others.
Now, go around the bike and unplug everything that can be unplugged in that circuit, except the instrument cluster (whose wiring is generally together with a bunch of other circuits that you need). Turn signals will have a connector nearby. Taillight will have a connector nearby. License plate lamp will have a connector nearby. Horn should have a couple of spade connectors right on it.
Switch everything off that is involved with that circuit and which has a switch ... turn signals, horn, brake lamp.
Now, the idea with the next step is to re-activate one circuit at a time, progressively checking its function, until you identify the moment the fuse blows. Option 1 is to do this with a number of spare fuses on hand. Option 2 is to do it with a multimeter set on measuring current (amps) on a scale that will allow reading up to 10 amps (normal capacity of that circuit). Either way ...
With everything unplugged, disconnected, and switched off in that circuit, turn on the key and verify whether the fuse remains intact / current reading remains low. The instrument lighting which is the only thing you didn't disconnect, should come on and stay on with the key as normal. If the fuse blows immediately, your problem is either in the instrument lighting or in the main harness up to the first (unplugged/switched-off) switching point in the circuit - that should narrow it down quite a bit.
Although nothing is plugged in at this point ... Activate the turn signals, first left, then right, then cancel them, then press the horn button, then squeeze the front brake lever, then squeeze the rear brake lever. The purpose here is to test the portions of the harness downstream of those respective switches. If the fuse blows when you activate a given switch, you now know that the fault lies in either the switch itself or in the portion of the harness between the switch and its respective load (connector).
If it survived that, plug in the taillamp connector. If the fuse blows, that's where your problem is. If it remains intact (instrument lighting remains on and your now-plugged-in taillamp works), squeeze the front brake lever, squeeze the rear brake lever. If the fuse blows when you activate any given switch, the problem is with the load operated by that switch (i.e. the taillight bulb or its socket or the pigtail-harness leading to it).
Then plug in the license plate lamp. There's no switch, so it should work. If it blows the fuse, there's your problem.
Then plug in one of the turn signals, and activate the turn signal in that direction. Yes, the flashing rate will be abnormal (or it may not flash at all) but our only concern at the moment is that the bulb works as opposed to blowing the fuse. If it blows the fuse, there's your problem. Same for the next one, and the next, and the last. Then plug in the horn, and check that function.
If it survived this process without blowing the fuse, then there may be an intermittent short, and these can be frustrating to diagnose. Throughout the above process, be on the lookout for insulation that is damaged, cut, or worn through. Pay particular attention to any place where a wire or harness is clamped to, or runs next to, any metal object.
wiring diagram, all electrical trouble-shooting starts there.