I believe it was 1967 that Sweden changed their traffic pattern from left to right hand. i read about it coincidentally earlier this week in an article talking about how New York City is considering adopting many Swedish techniques for reducing traffic accidents (an approach I am somewhat dubious about). They noted that the change itself reduced accidents, but are not clear why, but in any event they noted that change.
Since Winter Solstice was shot before then, they drove on the left side. There are also a number of shots of driving in Wild Stawberries and The Magician that also predate the change.