It's the Girl in the Red Truck, Charlie Brown(1988)
Charlie Brown and Snoopy get a letter from Snoopy's brother, Spike, who tells them that he has a crush on an aerobics instructor named Jenny who drives a red pickup truck.
This CBS TV-Movie was unique in that Charlie Brown, Snoopy, and Spike were animated while everyone else was live action! Creator Charles Schulz' daughter, Jill, played Jenny. The film was officially released on video, but hasn't yet been released on DVD.