Depending on whether you have the standard model or the ABS/CBS/TCS model - there may be just one fuse or two fuses supplying the headlights
The only ways that I can think of to can get a burned out fuse like that :
1. One of the wires between the fuse and the headlight / headlight relay has been rubbing and worn away the plastic insulation and contacted the frame. The offending wire will also be blackened / burned at the point where it made contact.
2. Someone has wired in additional components and decided that the existing headlight wiring is a good place to connect to. It isn't.
Some modles have two 10A fuses for the headlights. The standard model has just one twenty amp fuse. I guess yours is that one because the fuse that you show is yellow = 20A.
Each headlight is probably 60W or maybe 60/55. Lets assume bothe filaments (high and low beam) are 60W. 60w at 12v require 5 amps current (5x12 =60) / Watts = volts x amps). So for both bulbs that is 10A. The cable needs to be able to carry 10 A. But looking at the circuit diagram, it shows that the 'dimmer' switch - as they call it - that switches from high beam to low beam also has an N position when both high and low beam are connected to the power. That is 20 amps. There is no more capacity on that circuit for any more devices to be powered.
What is possible is that someone has tapped into the headlight circuit for the extra lighting and the circuit cannot handle that.
Alternatively, some of the connections are corroded. That adds resistance to the circuit so it needs more power. The wiring/fuse doesn't have the capacity so in this case the demand is not a sudden surge, but a gradual increase during which time the fue has been on its limit and running hot, not quite enough to melt the fuse wire.
Sometimes in order to prevent a fuse blowing people have been known to add extra equipment to existing wiring, and then add a bigger fuse to stop the original one from blowing. That is a disaster waiting to happen, because then, instead of the fuse blowing, the wire gets overloaded and the wire runs hot. The fuse cannot blow because it can handle the current, but the wire can't. The heat in the wire is conducted through the blades of the fuse and the palstic melts.
I'd do what
@ST1100Y said and inspect behind the fuse box and try to check the wire elsewhere.
Also try to find where the accessories are connected. It should be easy if you did it, and if you didn't it will alwo be easy because whoever did it didn't know what they were doing !
