Workers at a General Motors assembly plant in northern Mexico have voted for a new independent union to represent them after casting off a collective bargaining agreement negotiated by an old guard union last year.

The vote among the roughly 6,500 employees of GM transmission and pickup plants in the northern Mexico city of Silao was a major test of whether a measure of freedom has come to Mexican labor practices.

Workers chose the Independent Union of Auto Industry Workers, known by its initials in Spanish as Sinttia, by a wide margin over two days of voting. Sinttia received 4,192 votes, putting it well ahead of three other labor organizations, the Federal Center of Labor Reconciliation and Registry said Thursday.

For almost a century, Mexican unions have been largely a sham, with sold-out leaders guaranteeing low wages that drained manufacturing jobs out of the United States Mexican auto workers make one-eighth to one-tenth of the wages of their U.S. counterparts, spurring a massive relocation of auto plants to Mexico.

Under changes to Mexican labor law required under the U.S.-Mexico-Canada free trade pact, workers can now in theory vote out the old, pro-company union bosses.