This is a basic/brief guide to LED operation. LEDs do not run on 12V. White LEDs usually around 4V. So for very small LED power loads a resistor reduces the voltage but produces a lot of waste heat. For a large LED load like a headlight a "switching power supply" is used. This means pulses of 12V is sent to the LED. This circuit switching on and off is what creates a wide band of radio noise that back flows through the 12V car system to the radio. If it is really bad it makes it to the car antenna.(like your shop light)
A well designed "switching power supply" includes chokes/coils/filters that dampen out it's noise. All metal construction around the"switching power supply" also reduces the noise.
As the "switching power supply" can be built to run at any frequency depending on circuit design and what parts are cheap a aftermarket filter can be a " hit or miss" fix. Most filters are tuned to one frequency and get expensive if they really work on any frequency.
I would suggest you look to reviews and ask around for your best choice.
DOT in USA and Canada do not allow the replacement of a light bulb in a lens with a different light source(bulb#, HID, or LED) without retesting and certification. So most aftermarket LEDs are not legal in north america for on road use.
Bottom line is if the LED light source is not in the exact same place as the bulb filament the light focuses differently. You may like this but on coming traffic on unlighted roads will curse you. It will change the height of the cut off point and the width of the beam.
Last edited by Z09B4U; 08-05-2019 at 11:57 AM.