The union men had watched the company take what they believed were dangerous safety and manpower shortcuts to lower costs and boost production.
Alienating bosses could make the men company pariahs. But they knew they had to sound the alarm. From 2000 to 2005, their memo pointed out, Toyota had recalled more than 5 million cars -- 36% of all sold vehicles, a rate higher than other companies.
Toyota's failure to act, the two-page notice warned, may "become a great problem that involves the company's survival." They added: "We are concerned about the processes which are essential for producing safe cars, but that ultimately may be ignored, with production continued in the name of competition." They presented the letter to management and held their breath. But they needn't have worried. Toyota never responded.
"They completely ignored us," recalled Tadao Wakatsuki, 62, a veteran assembly line worker who formed the union. "That's the Toyota way."
Over the years, even before the recent worldwide recalls, Toyota was warned about declining product quality and worsening working conditions at its Japanese plants.
The warnings came not only from Wakatsuki's union, but from the widow of a 30-year-old Toyota worker who dropped dead at his desk and from an auto industry activist known as the Ralph Nader of Japan.
In 2008, the National Labor Committee, a U.S. human-rights advocacy group, released a 65-page report titled "The Toyota You Don't Know," detailing what it alleged were serious human-rights violations.
The report linked Toyota to human trafficking and sweatshop abuse in connection with its importing of foreign guest workers from China and Vietnam to work in its Japanese factories.
Many are pressured to work overtime without pay, the report claimed, adding that there were signs similar practices were emerging in the United States.
"Toyota is imposing its two-tier, low-wage model at its nonunion plants in the south" of the U.S., the report read, "which will result in wages and benefits being slashed across the entire auto industry."
Toyota officials said they could not confirm they received the memo but declined to comment further.
"Communication is the backbone of our labor-management relations," company spokesman Paul Nolasco said in Tokyo.
Assembly line worker Wakatsuki has seen what he calls the deterioration in working conditions and product quality.
In an interview, he listed a litany of concerns, including outsourcing key design work and shortening the trial-and-error period for new cars.
"We used to test every one of our cars for safety and quality," said the rail-thin Wakatsuki, a 45-year Toyota veteran. "Now we do maybe 60%. The old 100% is a thing of the past."
Believing that Toyota's unions were too compliant with management, Wakatsuki in 2006 formed the All Toyota Labor Union, which opened its ranks to contract and part-time workers ignored by the major labor group.
He created a website to publicize his views. Then, in the fall of 2006, six founding members drafted a memo warning Toyota about an impending disaster. After consulting technicians around the company, they also provided a detailed plan of action.
"Our responsibility as a labor union was to point out these problems that Toyota should have known about. People were overworked; some were committing suicide," he said. "Of course, Toyota did nothing, but looking back we see how important this was. We just told them what we saw."
Hiroko Uchino's complaint with Toyota wasn't over its products. She believes its workplace environment killed her husband.
In 2002, at age 30, the father of two collapsed at his desk of sudden heart failure. It was 4:30 a.m. and Kenichi Uchino had finished his assembly line shift hours earlier. But as a team leader, he was responsible for completing his paperwork on his own time.
The pattern had long concerned Uchino, who routinely worked 14 hours a day. In his final month, his wife says, he worked 144 hours of unpaid overtime, a common practice known as "service to the company."
In 2007, a Japanese court ruled Uchino had died from karoshi -- he had literally worked himself to death.
In an interview, Hiroko Uchino described the pain of watching her husband's energy ebb, how he went right to bed after work rather than play with his children or wash and wax his precious van.
"He used to tell me 'I'm tired, I'm tired, but what can I do? There's no way out.' He used to be such a happy person. But in the end, he stopped smiling," she recalled.
Two years ago, Uchinosneakedinto a Toyota stockholder's meeting to confront then-President Katsuaki Watanabe.
During a Q&A session, she got her chance: Without mentioning her husband by name, she challenged Watanabe about unpaid overtime.
"He didn't have an answer," she said. "He just turned to an underling and said they would look into it."
Nolasco said the company takes the death serious and said Toyota was "committed to strengthening measures meant to prevent work-related injury or harm" but did not elaborate.
One of Toyota's most vocal critics is automobile consumer advocate Fumio Matsuda, often called his nation's Ralph Nader, who in 1970 formed the Japan Automobile Consumer Union.
Now 84, the former Nissan quality control engineer has spent decades monitoring Toyota and Japan's other carmakers.
He calls Toyota's business practices the most secretive of all.
In the past, he said, Toyota sponsored "secret recalls," asking owners to visit dealers for vehicle checkups, a ploy that allowed them to replace defective parts and then charge the owner for the work.
"Everything Toyota does is hidden," he said.
Matsuda said he believed that Toyota also knew of defects involved in the most recent recall long before going public.
"I believe there will eventually be criminal charges," he said. "They knew there were problems with their cars, but they didn't do anything until they were pressured."