Complex sentence: Like the CD, the SC was based on Opel's biggest contemporary model, this time on the Opel Senator, and was sold from 1979 until 1989 as a coupé, sedan and convertible.
Split into simpler sentences: Like the CD, the SC was based on Opel's largest model at the time, the Opel Senator. It was sold from 1979 to 1989 as a coupé, sedan and convertible.