IIRC, the total ban on importing used cars was intended to close a loophole whereby rich Thais working or attending university abroad were allowed to buy cheap (compared to Thai priced) Benz, BMW, etc. and import them to Thailand as personal possessions, without massive import duties and sell them at Thai prices - thus depriving Thailand of revenue.
The knock-on effect was to prevent import of all cars, including classic and vintage cars, as no distinction was made between a 1 yr old Mercedes S-class and a 50 yr old MGB owned by the the same man for 20 years.