Germany (and most of Europe) is different. Everything you do to a vehicle in the aftermarket is supposed to be certified by TUV. I don't deal with motor vehicle parts but I do deal with all sorts of other equipment (mostly industrial automation), some of which is either coming from or going to the EU.
If you are the buyer of the parts and the owner of the vehicle in question, then the manufacturer of the parts in question should be providing a "declaration of conformity" which shows that the parts meet the applicable standards in question. Keep this with your vehicle and when it is time for your periodic safety inspection by TUV, have them available to show to the inspector.
If you are the manufacturer/distributor of said parts and are selling them to others, then you're the one who has to establish which of the motor vehicle standards are applicable to the parts in question, establish that the parts conform to the applicable standards, and provide that written declaration of conformity stating thus - and you have to take responsibility in the event of any non-conformances discovered afterward. The standard may require specific markings on the parts themselves.
So, which situation is it?
