Lakedude, they could both be right, if the actuary guessed wrong 25 years ago, and "bet" -medical costs would only rise at the rate they had been rising, then the fund would have been "fully funded" at the time, but it would now require an injection of current workers money.
Martinjlm didn't say"they are not a burden on current employees" you said that. He said-in effect-that the plan was fully funded at the time according to the best guess of the actuary who did the "guessing".
This could be part of GMs problem-a problem Toyota doesn't have, or doesn't have much of since they have very few retired USA employees.
GM might have fully funded the fund at the time-according to a "best guess" of medical costs. It almost certainly would have been an underestimation of medical costs since medical cost increases have continued to exceed expectations.

They can both be correct.
Fully Funded doesn't mean they aren't a burden on current employees.It means-probably-that it wasn't meant to be a burden on current employees.
Luck,Charlie
PS Healthcare costs might do what Hillary couldn't-get universal healthcare.