Perhaps, just perhaps, the positions aren't worth the same hourly rate they once were?  Once, building cars and trucks was a very dirty messy dangerous occupation, lots of injuries and almost continuous repetitive movement as the line went past.  A lot of the lifting has been replaced by robotics, the level of skill has increased perhaps, but a significant drop in the number of employees required to build each vehicle.

One might suggest that moving forward, the required number of person hours necessary to assemble the vehicles will decrease not increase, again lessening the number of employees per shift.

Perhaps the good folks at Unifor might look at organizing the Robotics or at least those who programme them.  Either that or start to unionize the Mexican Autoworkers.