Engines and tires are the pieces they are supposed to be looking out for, due to emissions of non US engines and road safety of non DOT approved tires. Practice may be very different on those items.
For small items coming through the mail, specifically lamps, there does not seem to be much effort to look for or at and/or inspect. It is much more likely for you to get off an international flight and find a TSA/Customs note inside your suitcase saying that they opened your baggage and rummaged through your underwear for the safety of your fellow air travelers...
With lamps, the problems are much more likely to come from annual vehicle inspection clerks and local law enforcement.
RHD headlamps "dip" the opposite direction, meaning they are brighter on the left where the shoulder is, in order to see pedestrians and obstacles off the shoulder side of the road, and dimmer on the right in order not to blind oncoming drivers. This is the opposite of US lamps. EEC/European lamps have a more drastic and less gradual transition in beam from bright to dim in the beam pattern, than US lamps, so the bright area is or appears to be larger and there is less visibility overhead and to the sides.
There is one gulf coast state that has equipped all their inspection stations with a headlight beam aiming device, and the inspection clerk will basically ignore items like brakes, cracked windshield, undercarriage dragging on the ground, and will spend fifteen minutes trying to fail every car due to headlight aiming or beam pattern problems.
There is a Great Plains state college town where one officer spends his night shifts ticketing drivers using driving lights, high beams, or lamps that are, in his opinion, too bright, and his day shifts ticketing drivers for not having a front license plate or plastic plate cover. His cousin, the judge, upholds the tickets.
Outside one Midwest city known for people murdering police officers, one highway patrolman recently took time out of the daily grind of looking for police killers to lecture me for an hour about my front license plate on my dash, because the mounting plates in my 20+ year old daily driver had pulled out of the bumper the day before, and I had the forethought to remove it before it fell off on the road, but had not yet figured out how to properly reattach it in the snow and ice storm we were having.
Lamps and visible stuff is more likely to bring you into conflict with local police and inspection clerks when they are on the car, than with Customs when they are in a package passing through the postal system.