Re: Oh the irony...
I assume it's part of FMVSS108, could you point me to the correct area?
It's further up the regulatory chain than that, because it applies to all regulated safety features, components and systems, not just lights. See 49-CFR-571. Basically it says that if a piece of regulated vehicle equipment (such as a headlight, a seat belt, a window, a sideview mirror, a tire, a bumper, etc.) is physically capable of being directly installed on a vehicle certified as conforming to all the applicable Federal standards, then that piece of equipment itself must conform to all the applicable Federal standards, and must be certified. There is no exemption or provision in the regulation for "off road" equipment. If it _physically_ fits, it has to conform. If it doesn't conform, NHTSA can levy large fines and recall orders. There's some grey area in the term "directly installed"; there have been some nonconforming headlight bulbs marketed with extra plastic or metal tabs on their bases to prevent their installation in a certified headlight (until the buyer clips off the extra tab). But there's also the prohibition on selling
unregulated vehicle equipment which, when installed, ruins the conformity of a regulated piece of equipment. For example, brake light bulbs aren't regulated, so the first test ("Does it directly fit on a certified car?") doesn't apply; there's no such thing as a certified brake light bulb. BUT, because the conformity of the brake light itself depends on the performance of the bulb, it's illegal to sell, import, introduce into interstate commerce, or (for money) install a bulb that doesn't work correctly, such as an "LED bulb" in place of a filament bulb. There's also a special section just for headlight bulbs (49-CFR-564), which is what nukes HID kits, legally speaking.
No, it's not Federally illegal to
possess an "HID kit", nor are vehicle owners regulated by the Federal Motor Vehicle Safety Standards or any Federal law pertaining to them. Rather, vehicle owners (and registered vehicles, etc.) are regulated at the state level.