Mexico's new-car and light-truck sales fell 10.7 percent in September from a year earlier, to 77,267.
In the first nine months of 2008, sales dropped 2.0 percent to 763,020.
In September, sales of vehicles made in Mexico rose 10.1 percent to 33,065, fueled by a 26.5 percent jump in domestic truck sales to 10,647. But sales of imported vehicles slid 21.8 percent in September to 44,202.
Nissan and General Motors remained the top two automakers in sales.
Nissan's September sales slipped 4.9 percent to 16,215, while GM's fell 15.4 percent to 14,304.
Volkswagen's 5.2 percent sales drop, to 11,473, put it ahead of Ford Motor Co., where sales slid 19.1 percent to 10,363.
Among major automakers, Toyota saw the largest decline in September sales, dropping 26.7 percent to 3,971. That put it behind Honda, whose sales rose 12.5 percent to 4,621.