It is not mandatory for a passenger car with a trailer to carry a spare wheel for the trailer. Here is why:

- **Spare wheel in the vehicle:** It is important to carry a spare wheel in the main vehicle, such as a passenger car, to ensure you can change a wheel in case of a puncture or damage. However, this obligation does not extend to the trailer.

- **Trailers and their classification:** Trailers are classified as light and heavy. A light trailer can carry up to 750 kilos, while a heavy trailer can carry more than 750 kilos. Despite these differences, there is no mention of the need to carry a specific spare wheel for the trailer, regardless of its type.

- **Safety checks:** Before a trip, it is essential to perform certain safety checks, such as carrying emergency triangles, a reflective vest, and a spare wheel for the main vehicle. There is no mention of the need for a spare wheel for the trailer.

Therefore, while it is crucial to keep the main vehicle in good condition and equipped with a spare wheel, this obligation does not apply to the trailer.