A voltage dropping resistor is the only way to keep a 6-volt design from sounding like a banshee for a high current draw component like a series wound horn motor. Perhaps the resistor has a bit too much resistance. Resistors make heat so that heat has to be disipated rapidly to keep the resistance within parameters. I would try a resistor that is made to drop voltage for such devices.
This link illustrates a dropping resistor mounted to a heat sink. other sources sell this type as well.
https://www.modelaparts.com/a13803