What must you check on a car with a caravan trailer before setting out on a journey?

In the context of traveling with a caravan trailer, there are specific checks that must be made to ensure safety and compliance with German traffic regulations.

1. **Lights of the Trailer**: It is crucial to check that the lights on the trailer are functioning properly. This is necessary for notifying other drivers of your intentions, especially when braking or turning. German traffic law requires that all vehicles, including trailers, have working lights to ensure visibility and safety on the road.

2. **Rear-View Mirrors**: The adequacy of the view through the rear-view mirrors is also essential. When towing a caravan, the dimensions and weight of the trailer can obstruct visibility. Properly adjusted mirrors are necessary to monitor traffic behind you and to change lanes safely. This falls under regulations about ensuring a clear field of vision for safe driving.

3. **Safety Belts**: While it is important that all passengers wear safety belts as part of general vehicle safety regulations, in this specific question, checking passengers in the caravan trailer is not among the primary checks before starting the journey. The focus here is more on ensuring that the vehicle itself is roadworthy and that the driver can operate it safely.

By combining the two necessary checks on trailer lights and rear-view mirrors, you ensure that you are setting out with a vehicle that meets safety standards and minimizes risk on the road. Skipping any part of this preparation could lead to dangerous situations while driving.