Here's a circuit that uses a 6V relay and a 12V relay without any electronics. The value of resistor R is selected to be approximately equal to the value of the two resistor coils in parallel.
When horn [1] button is pressed, relay K1 is pulled in and removes the power to relay K2. When horn [2] button is pressed, relay K2 is pulled in through the resistor. This keeps the relay voltages at around 6V so that K1 does not pull in.
You may have to play with the value of R some to get it to work properly, depending upon the relay characteristics.
For best results the two relays should probably be of the same design and manufacturer, differing only in coil voltage.
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