Bulbs blowing is an indication of over voltage. The rectifier for the horn only changes your AC to DC (and likely only halfwave at that).
If you look at a standard rectifier/regulator the case will usually have some finning on it. That finning sheds the heat produced when the regulator clamps peak voltage.
It's like the zener diode on old british bikes. They were mounted in a big finned heatsink that is out in the airflow. You've probably seen them under the fork crown on a BSA. The excess electrical energy is converted to heat which has to be disspated.
You definitely need to add voltage regulation. A TrailTech unit would work well, but any combination reg/rect (single phase) should do.
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2011 EC250E
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