AMDA charges that many of the imports are wrecks and do not meet the safety and emissions standards to which new and used cars sold in Mexico are subjected.
The decree allows secondhand vehicles as heavy as 4.5 metric tons (5 tons) to enter the country legally, but they must be at least 10 years old and no older than 15 years. AMDA alleges that importers often bring in newer vehicles and declare them to be 10 to 15 years old.
The decree, which was signed by President Vicente Fox before he left office, did not require congressional approval.
It apparently was designed to boost the standing of Fox's National Action Party among millions of poor Mexicans before the presidential election of 2006, which the party won by a narrow margin.
AMDA alleges unfair competition. It says nobody checks that the imported vehicles are safe to drive and that they meet the anti-pollution standards required of new vehicles.
Another irritant for AMDA is that a value-added tax of 4.5 percent is levied on imported used vehicles, but the value-added tax on new cars sold in Mexico is 15 percent.
The leaders of AMDA, which has 1,600 members that constitute 97 percent of the country's dealers, are understandably worried and angry.
Association President Jose Gomez says the imports have hit Mexico's subcompact sector the hardest. That is traditionally Mexico's largest new-car segment.
Subcompact new-car sales fell 20 percent in 2006 and are likely to dip another 20 percent this year, Gomez says.
According to AMIA, the national manufacturers association, through the first 10 months of this year, sales of all Chrysler-brand light vehicles were down 0.4 percent from the same period in 2006. Ford-brand light-vehicle sales fell 14.8 percent; Renault's, 8.0 percent; and General Motors', 5.7 percent.
Light-vehicles sales for the Peugeot and Volkswagen brands also were down slightly, and Nissan's were stable. None of the makers of cars for the cheap end of the market reported a gain.