What was the ending to ';Repo Man';?

I really like the 1984 cult classic Repo Man but I was slightly confused with the ending where Miller gets in the UFO/Chevy Malibu and takes Otto for a ride through the sky. Was Miller an alien all along, did he just somehow know how to drive a flying car, or is it open to interpretation?What was the ending to ';Repo Man';?
Whatever was in the trunk is what made the car fly. Its kind of like what was in the briefcase in Pulp Fiction. They never really said.