I think you are in
and the remaining one is
I have a mountain named Ul, where cars are born.
Ulsan - where "san" is Korean for mountain and Ulsan has the world's largest automobile assembly plant operated by the Hyundai Motor Company.
I have a great farmland, where the cars cross.
Daejeon translates as "grand field" and is at the crossroads of the national road transport routes.
I have a mountain of cauldrons, where ships are.
Busan translates as "cauldron mountain" and is the largest port city in South Korea.
I have a benevolent river, where airplanes are.
Incheon translates as "kind river" and Incheon International Airport is South Korea's primary international airport.
I have a king, who is one of two "great"s.
Sejong was named in honour of King Sejong the Great. (I think the other "Great" was Gwanggaeto)
I also have a great hill
Daegu - whose name means "large hill".
and a province of light.
Gwangju literally translates as Province of Light
There is the remaining one that I haven't mentioned.
These are 7 of the 8 first-level cities in South Korea. The only one missing is Seoul, the capital.