williamr wrote ...

LEDs offer a much higher resistance than incandescents, so a resistor in parallel between the lamp and ground is needed to convince the computer that the bulb hasn't blown.

Resistor value depends on the wattage of the lamp, but it's easy to calculate. Assume that the LED has infinite resistance and just fit a resistor of the same value as the stock bulb using R= V^2/P, or crudley, resistance (in ohms) = 144/the power in watts of the original bulb.

Make sure that the resistor has the same power rating as the original bulb, and mount it where it can dissipate the heat that it will throw off.

For turn signals, getting the wrong value will alter the flash speed. Scope there to play around.

For the headlamps, Phillips Xtreme vision are very good, but tbh, I find the stock headlamps fine.

Higher power than stock, or HID conversions, are illegal and potentially dangerous, but fitting the Citroen HID lights is OK, if expensive.

Rob