If the start button applies power to the motor and the stop button removes it, then simply connect the bulb in parallel with the motor.
This will only work if the bulb is rated for the particular voltage.
If the bulb will burn out due to too much voltage, or if you think it might, you could power a relay with the motor's power source, and have a second lead from the power source to run through the relay to power the bulb, and reduce the voltage to an appropriate level via use of resistors.
Find the difference between the power supply's voltage output, and the optimum voltage for the bulb. Take the difference, divide that by the amperes the bulb will draw (bulb watts/bulb voltage), and that will tell you the resistor you'll need to use.
(PS.V)-(B.V)/(B.W/B.V)=R
PS=Power supply
B=Bulb
V=Voltage
W=Watts
R=Resistor needed