Using diodes to drop voltage is smart, since there is hardly any power loss to heat.
Using a "smaller fuse" won't work, since a fuse is a safety device, not a voltage dropper. The fuse element melts in half to protect from over-current. That causes an open circuit. Lights out..
A long thin "feed wire" would work. If it were long and thin enough to drop 3.5 volts,
the remainder of the voltage would be applied to the headlamps.
BUT, the wire would get warm. 3.5V x ~10 Amps =35 watts of heat. Wasted heat.
~10A is assuming 2 lamps at 5A each. 13V x 5=65W lamp..
I have no idea what wattage headlamps are used these days.
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