A shunt R for your application would have to be very low if applied across
the LED with its resistor. You could use a higher value if you could open
the light casing and place it in parallel with the LED proper. Yet another
solution is to connect a LM317L with a single resistor as current source
in parallel with the light as is. Use the LM317 data sheet to select a
resistor to set a 2mA current through it. There are other solutions ...

Peter

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