Light-emitting diode (LED)...diode means current
will flow in only one direction through the device.
If the light is not ON the possible causes are:
- the LED is defective
- the LED is not getting the proper voltage (open wire or high resistance connection or voltage too low)
- the voltage polarity is reverse of that needed to flow through the diode(s)
If the polarity is reversed the diode will not be damaged (unless the voltage applied is high enough to cause a breakdown of the diode...much higher than your 12V supply).