MEXICO CITY (AP) — Mexico has faced its biggest challenge yet to labor-freedom reforms under the U.S.-Mexico-Canada trade agreement: An old-guard union was caught apparently destroying negative ballots at a General Motors plant in northern Mexico. Labor Secretary Luisa Maria Alcalde says the vote tampering at the pickup truck and transmission factories in the northern city of Silao were a sign that some union leaders “haven’t gotten the message” that they can no longer operate behind the backs of workers. The vote was a test of Mexico’s commitment to enforce re-votes on union representation required as part of the trade agreement, known as the USMCA.