12volts down to 6volts

[Pep]

Use a multimeter and test across the horn's terminal. Whatever the reading is ( and I suspect it will be fairly low about 10 ohms or so ) Then get a resistor the same and put it is series with one terminal. Make sure its at least 10 watt rating. Some heater fans use resistors around that size as well.

Correct Pep, however I doubt 10 watts would be anywhere near enough.
To get the correct wattage resistor, connect the multimeter in series with the horn on a 6volt supply using the Amps reading, and take a measurement.
If its using 4 Amps for example (and this is still probably conservative), use simple electrical theory as follows. Power (watts) = Volts x Amps. In this case it would be 6 x 4 = 24watts, and thats the resistor's rating you'll need to dissipate exactly half of the power through it. So make sure you get one suitably rated, to the same resistance as determined above in Peps post.
I suggest using the larger size simply due to the horns startup current is much higher than the running current, and its required to dissipate much more energy for the first 1/4 second than it normally will during use and will prematurely burn out if you dont use the right one.

Since it's not continuous, then 10 watts should be OK. It will get hot if you lay on the horn for a minute, but the occasional burst will be OK.

Santa Fe Bri, As far as the wattage rating, that's not the issue. The resistance value is the important thing. The wattage rating is just how much power the resistor will take before it fries. Also, the Klaxon is a motorised device. It will go better if its cleaned and lubed in the bearings. There is an adjustor in there as well. You adjust that while its running for the desirable sound.

[Santa Fe Bri]

Thanks for all your input guys.its been fun...............Cheers Bri