The **rear fog lights** are essential for improving visibility in adverse weather conditions. They should only be used when visibility conditions are very poor, such as in the case of a **heavy snowfall**. This is because intense snow can significantly reduce visibility, making it difficult to see other vehicles on the road. Rear fog lights help make your vehicle more visible to drivers coming from behind, reducing the risk of accidents.

On the other hand, when driving through a **tunnel**, the vehicle's lights should be turned on, but not specifically the rear fog lights, as tunnels are usually lit and do not present the same low visibility conditions as a heavy snowfall. Additionally, when driving through a **mountain pass**, it is not necessary to use the rear fog lights unless the weather conditions require it, such as in the case of dense fog or heavy snowfall.