I would put a dimmer on any supplemental lighting in the cockpit. The amount of light needed may vary depending on how close you are to sunset/sunrise as well as ambient lighting from populated areas.
I would not buy a LED-light without a specification.
You need to know the Voltage and the Current draw.
Be sure the lights are recommended for dimming.
A dimmer can be of the resistivity type (ohm) or a PWM-dimmer.
A good PWM-dimmer will not cause noise in your hed set, but a cheap one may
cause problems. It´s important that the LED will work as intended with the dimmer chosen.
On the 12iS, Van's uses a potentiometer for dimming the LC ECL-02.
They don't carry this potentiometer in their store, but I was told the P.N. is a Bourns 53RAA-R25-A18L. I picked one up from Arrow.com. It was $2 for the part and $7 for shipping.