Complex sentence: Designs of the XJ date back to 1978 where a team of AMC and Renault engineers done several sketches; a few clay models were based on the existing SJ Cherokee.
Split into simpler sentences: Designs of the XJ Cherokee date back to 1978 when a team of American Motors (AMC) and Renault engineers drew several sketches. A few clay models were based on the existing SJ Cherokee.