If you've got metter that is a VOM Witch is a Volt , Ohm Metter. Or will purchase one from a cheep sight. I can take you thru a equation that will allow you to purchase a or and resistor to do this a lot simpler.
What you'll do is to figure What The resistance is buy measuring the Hot to Neutral and blow the horn with the meter set to a r or ohm value .
Now you got the value of Ohms or the resistance of the circuit. At 12 volts
If you will multiply the Ohms by 2 you'" get a figure for resistance at 6 Volt.
Now go buy a resistor that droups the ohms but this much
