Try this resistor
https://www.speedwaymotors.com/Electric-12-Volt-to-6-Volt-Reducer-for-Motors,15824.html?sku=91064103&utm_medium=CSEGoogle&utm_source=CSE&utm_campaign=CSEGOOGLE&gclid=EAIaIQobChMI0YfZiaq12QIVkVp-Ch3YIQTgEAQYAiABEgIZS_D_BwE
It's what is commonly used on 6-12v blower motors.
A good place to mount it would be next to the horn for heat dissipation.
Resistor