Opel cars appeared under their own name in the US from 1958 to 1975, when they were sold through Buick dealers as captive imports. The best-selling Opel models in the US were the 1964 to 1972 Opel Kadett, the 1971 to 1975 Opel Manta, and the now-classic 1968 to 1973 Opel GT. (The name "Opel" was also applied from 1976 to 1980 to vehicles manufactured by Isuzu (similar to the "Isuzu I-mark"), but mechanically those were entirely different cars).
The Kadett C appeared in August 1973 and was Opel's version of the General Motors' "T-Car". It was the last small Opel to feature rear-wheel drive, and remained in production at Opel's Bochum plant until July 1979, by which time Opel had produced 1,701,076. Of these, 52% had been exported outside West Germany, most of them to markets in other parts of western Europe.
It had some crazy mechanical issues. At one point it stumbled and stalled because the carburetor had come loose from the intake manifold and I had to tighten it back down! Another time, my brother was driving it and the brakes failed. Somehow the end of the brake shoe that meets the brake cylinder piston had become worn on one side, causing the piston to slip off the end of the shoe when the brakes were applied, losing braking and squirting fluid all over.