Even though I couldn’t access any official car sales data for the period in Sweden, I will venture an estimation: the Volvo Amazon (aka 120 Series outside of Sweden) reached a production figure of 667,323 units at the end of its life in 1970. 60% of these were exported, which means it sold on average 19,000 units per year in Sweden. This should be enough for it to lead car sales in Sweden once the Volvo PV started to decline, so possibly from 1965 to 1967. The Volvo 140 Series then took the relay from 1968 onwards.
In parallel Saab launched the 96 in 1960 and 550,000 units of the model were produced over 20 years, making it an average 15,000 sales in Sweden in the 1960′s. The Saab 96 should logically rank #2 in the country during the period.
If you do have any official car sales data for the period, please make sure to get in touch!