If your horn is 6 volt you can make a voltage dropper as follows:
A silicone diode is a check valve for electricity. It also drops the voltage 0.7 volts. If you go to radio shach and get nine of these and hook them up in series then you will drop the volatge 6.3 volts across the nine diodes, and 7.3 volts across the horn. Mos alternators put out about (13.6 volts.). You don't need the check valve function, just the voltage drop.
This is easier than using a resistor because the drop across a resistor is current dependent. Drop across the diode is not, so no calculations are required.
If you have a volt-ohm-amp meter then use the meter to measure how much power the horn takes, using your lantern battery. Make sure you get diodes rated for more than the horn consumes.