It would be easier and cheaper to use a resistor,
As the decoder puts out a constant 12 volts dc, use Ohms law to calculate the value of a RESISTOR to place in series with the 1.5 volt lamp.
You will need to know the current draw of the lamp. Then use the formula R=E/I, where R=Resistor required, E= Voltage drop required (10.5) and I= Current draw of the lamp.
Wattage rating of the resistor would be Current times Voltage Times 2 for safety.