comments:
My process for this would go something like this: Use a variable voltage source (like a benchtop power supply) to find the LED dropout voltage. Determine how much current the LED draws when running. A DMM with a 10A current input is probably the correct tool for this. Use an oscilloscope to determine the frequency of the voltage variations. Using the frequency as the time constant and the LED current to determine the overall resistance, calculate the capacitance using a tool like this. Installing the appropriate amount of capacitance will likely cause problems elsewhere on the vehicle though, for several reasons. I'd suggest trying a wire-wound resistor in parallel with the LED. Something in the 25ohm range like this. It puts some additional load on the circuit, which often resolves the flicker. You can test this quickly by wiring up the old light in parallel with the new one. If the new one stops flickering, that's your solution.