When the brothers spontaneously buy the red convertible, it symbolizes their youth and freedom. It symbolizes unbounded opportunity and optimism. At the end of the story, when Lyman releases the clutch to let the car roll into the flooded river, it symbolizes lost youth, lost love, lost brotherhood, lost hope and lost opportunity; it symbolizes lost chances.
Even though the red convertible is bought with money from two disasters (part of Erdrich's extensive foreshadowing)--the loss of Lyman's Joliet Cafe and the loss of Henry's job ("Henry had two checks--a week's extra pay for being laid off, and his regular check from the Jewel Bearing Plant")--it still symbolizes optimism and opportunity for them. In their youth and enthusiasm, it symbolizes to them--they feel it as symbolizing--a promise of future opportunity and good luck. They see it as a living thing that promises reward and joy: "We went places in that car, me and Henry."
There it was, parked, large as life. Really as if it was alive. ... That car reposed, calm and gleaming .... Then, before we had thought it over at all, the car belonged to us....
After Henry returns from Vietnam, torn up and agitated psychologically and, in some ways, physically (a condition represented and symbolized by the lip he bit through), the red convertible symbolizes both lost hope and youth and, ironically, a return to hope and youth. Of course, Lyman treated the red convertible in a disastrous way to manipulate it into a vehicle of hope for Henry. Lyman hoped his actions might make this object from Henry's loving, active, happy past into the sort of object that might absorb Henry's horrible thoughts and engender life-loving, new thoughts.
When Lyman and Henry get to the river in the red convertible, it symbolizes the power of a present trauma over a past life of love. On the drive, Lyman thinks he sees "calm, more peaceful" thoughts echoed on "his face," thoughts of "bare fields and windbreakers and houses." By the riverside, Lyman knows "the squeezing and tightening" he feels is how Henry feels inside: "I knew I was feeling what Henry was going through." Unable to "stand it," Lyman jumps up and shakes him by the shoulders yelling, "[W]ake up, wake up, wake up!" Here the red convertible symbolizes failure, lost chance and lost hope.
[Henry's] face was totally white and hard. Then it broke, like stones break all of a sudden when water boils up inside them.
"I know it," he says. "I know it. I can't help it. It's no use."
We started talking. ... He said he'd fixed [the car] just to give it back [to me]....
When Lyman sends the red convertible into the river after Henry (having failed in the swollen springtime river--with its strong, pulling current--to save Henry, whose boots filled with water), it symbolizes the lost love of a brother drowned, lost hope of reclaimed lives, lost opportunity and lost chances. It symbolizes lost luck and lost optimism about life and living.