Basically importing used vehicles is not allowed except transiting to the other countries.
The Customs Department
http://www.customs.go.th/Customs-Eng/indexEng.jsp
The Excise Department http://www.excise.go.th/eng-index.html
Department of Foreign Trade (only Thai)
http://www.dft.moc.go.th/index.asp?solution=800
Royal Thai Embassy http://www.thaiembassy.jp/rte2/