The lights on a trailer must be electrically connected to the corresponding lights on the truck.

Any ABS brakes on a trailer must be electrically connected to the electrical system of the towing vehicle, usually through a special electrical connector.

Legal regulations on mandatory lights and reflectors

A trailer may only be equipped with mandatory and permitted lights and reflectors.

Mandatory lights and reflectors include:

• Two lights with position lights. (If the trailer's permissible total weight does not exceed 3500 kg, these are only required if the width exceeds 1.6 meters).
• Two white forward-facing reflectors.

• Yellow side reflectors placed at appropriate intervals.
• Yellow side marker lights if the trailer is longer than 6 meters (the coupling device is included in the trailer length). Older trailers (1996) may be equipped with either side reflectors or side marker lights.

• Two lights with tail lights.
• Two lights with stop lights.
• Two turn signals at the rear.
• At least one license plate light must be readable from at least 20 meters away.
• Two red rear-facing triangular reflectors positioned with the tip facing upwards.
• Yellow reflective plank(s) with red border.

Trailers wider than 2.1 meters must also have two front and two rear marker lights, white and red respectively, that can be clearly seen at a distance of 300 meters without glare.

Position lights must be clearly visible from at least 300 meters away without glare.

Rear lights must have a red light that is clearly visible at a distance of at least 300 meters without glare.

Yellow side reflectors that must be approved and distributed on the sides of the trailer. On older trailers (1996), side reflectors can be replaced by yellow marker lights that are clearly visible at a distance of at least 300 meters without glare.

Trailers must always be equipped with side reflectors and, if longer than 6 meters, side marker lights (side reflectors and side marker lights may be combined).

The coupling device must be included in the length of the trailer vehicle.

Legal regulations on permitted lights and reflectors

The permitted lights and reflectors include:
Tail lights, work lights, auxiliary tail lights, auxiliary forward and rearward facing marker lights, auxiliary side marker lights, rear fog lights, side turn signal lights,
front indicators, additional rear indicators, additional stop lights, hazard warning lights and additional reflectors.

Trucks/trailer combinations can be equipped on the sides with a reflective contour marking in yellow or white and advertising or logo in a low-reflective material that can be in any color. The rear of the truck/trailer combination can also be equipped with contour markings in red.

Checking lights etc.
All lights must be able to light up.

Lights in light pairs must be of the same color and strength.

Lens and reflectors must be clean and intact.

Stoplights must provide significantly brighter light than taillights.

The indicators must flash with a yellow light that is clearly visible in sunlight.

License plate lights must have white light that can illuminate the license plate. Mandatory reflectors must be approved.

Rear reflectors must be positioned with the tip facing upwards.

Signs of failure:
Lights that do not work or work incorrectly may be due to a fault (poor connection) in the electrical connector between the vehicle and trailer.

If the indicators are flashing significantly faster than usual, it is usually a sign that one or more indicators are not working.