My memory from the production in London, which I saw in June… I thought Marty was playing with his band. Doc interrupts, the “concert” set rises, car is on the stage, basically Doc says the line you mentioned, they both get into the car, car flies into the audience, does a barrel role, turns around and goes back into the stage, as the ensemble sings off stage, and music swells, car passes the proscenium, that front scrim comes down, projection “The End”.
My memory is already iffy, but I thought it was the clock tower check scene, then concert, into car finale. Maybe it’s the reverse? … maybe I can’t remember anything anymore 