Complex sentence: These included Fisher Body, Pontiac East Assembly (a.k.a. Truck & Coach/Bus) which manufactured GMC products, and the Pontiac Motor Division, which in the city's heyday was the primary automobile assembly plant where the famed Pontiac cars were produced.
Split into simpler sentences: These included Fisher Body, Pontiac East Assembly (a.k.a. Truck & Coach/Bus), which manufactured GMC products, and the Pontiac Motor Division. In the city's heyday, this was the primary automobile assembly plant where the famed Pontiac cars were produced.