Complex sentence: BMW (Werk Eisenach), Hanomag, Stoewer, Opel (Werk Brandenburg), Ford Germany and Auto Union (Horch und Wanderer), who themselves in turn outsourced a large fraction of the individual parts to third-party suppliers.
Split into simpler sentences: BMW (Werk Eisenach), Hanomag, Stoewer, Opel (Werk Brandenburg), Ford Germany and Auto Union (Horch und Wanderer). These companies in turn themselves outsourced production of a large fraction of the individual parts to third-party suppliers.