Complex sentence: All 1969--1970 Shelby Mustangs were produced in 1969 as sales were falling down and 789 remaining 1969 cars were titled as 1970 models.
Split into simpler sentences: All 1969--1970 Shelby Mustangs were produced in 1969. Because of dwindling sales, the 789 remaining 1969 cars were given new serial numbers and titled as 1970 models.