Complex sentence: They moved to Pontiac in 1959 to produce the artwork for the Pontiac "wide-track" campaign, which concluded in 1972, the longest, and arguably the most successful, ad campaign in American automobile industry history.
Split into simpler sentences: They moved to Pontiac in 1959 to produce the artwork for the Pontiac wide-track "campaign, which concluded in 1972." Wide-track was the longest, and arguably the most successful, ad campaign in American automobile industry history.