There is alot going on.
1. They don't have the entire US market to themselves so if the union says we want another $2 an hour, Ford can't raise thier prices on thier vehicles and stay competitive. The market alone keeps eating away at thier share, the best thing they have going is the world market and thier cheap prices.
2. Thier quality has traditionally been bad. They make some good cars and boast about the quality, ie the Taurus, and then they make some bad cars that turn people away again, ie Explorer and Windstar. They are much much better than they were in the early 80's, but definetly not Toyota-Honda quality. People don't trust them.
3. The union is a HUGE burdon, labor they can't get rid of because of contracts, every company shrinks once in awhile, but Ford-GM can't get rid of half thier workforce when they need to. The labor force is much older than Toyota-Honda, they have huge pensions to pay to the overpaid employees and with people living longer have more and more expenses.
Lookup sometime what kind of benefits a UAW member gets and you will be sick to your stomach, they took advantage of what they had, and now the American auto industry is suffering. Even when GM, Ford, Daimler-Chrysler and Delphi basically have them by the balls they still ask for the huge wages, health care, 22 holidays (estimated), vacation and whatever else. Did you know if a membor gets laid off, they have a 1 year job bank that pays them normal wages even though they are not working, plus they are guarenteed a job if something opens (so much for a younger workforce coming in). The best thing they can do is to buy them out, close plants and make the UAW as weak as possible before they can expand in the US again.