Can you be fined for not wearing seatbelt in old cars?

It is fairly rare now that cars are not equipped with three seatbelts in the back, but if you have an older one with only two, it is true that a third passenger in the back sitting in the middle would have no belt.

Firstly, owners of old cars that do not have three in the back are not legally obliged to modify them to install one. Secondly, the law says that where there is a seatbelt it must be used. But since you cannot use a seatbelt that does not exist, then no there should be no fine for not using one if sitting in that place. However, it is forbidden for a child under three to be travelling with no seatbelt even in that situation.

On the other hand, in cars which do have three back belts, there is a €90 fine (€135 if not paid promptly) for not wearing one, which is payable by the driver if the passenger is under 18.