I doubt they have one wiring harness for both cars as that would have a lot of redundancy (and cost) built into it for lower spec cars which is generally the majority of cars sold. As an example relatively few cars have the factory fit rear DVD player....I am almost 100% sure that the redundant wiring is not their on cars that dont have it fitted.

If BMW can extract cost savings by only having no redundancy in the cars they would definitely do it. Whoever supplied BMW with the wiring harness will have all the problems of supplying wiring for all the different specs.