I'm wondering something? When you take the fuse out after it blows, is it just burnt or is it splashed (by that I mean it looks like an cartoon gunshot)?
The reason I ask that is if it is splashed that is a clear indication of a dead short. That is what I suspect is happening to you that there is a dead short through that circuit once it's enegized on startup. Sometimes a loose ground will cause a fuse to blow also, as well as a loose fuse holder. If you don't have a manual try to get one & follow the circuit to see how it runs. Remember a wiring scematic is really nothing but a road map, the cross connections are intersections, contacts/switches are only bridges.
I'm sure one of the wiring gurus will chime in with check relay 3 & etc etc etc. However hopefully this help in the meantime.
I would check all the wiring connections of the circuit for loose connections & if there are relays in the circuit check the contacts if visible. It sounds as if that may be your issue. As always this is just One Ole Fossil's opinion.
Thanks for your advice. After trial & error,6 more fuses it turned out the plastic covering the inside of the switch module?? had worn enough that it was shorting out on the handle bars. A small piece of electrical tape seemed to have solved the problem.