Complex sentence: In 1990, Prodrive began a long standing and highly successful relationship with Subaru, running their rallying campaigns and helping them to 3 Drivers'and 3 Manufacturers' World Rally Championships.
Split into simpler sentences: In 1990, Prodrive began a long standing and highly successful relationship with Subaru. Prodrive was tasked with running Subaru's rallying campaigns, helping them to 3 Drivers' and 3 Manufacturers' World Rally Championships.