Ultimately, it's impossible to know without being inside. I would imagine there's a fair bit of reuse within each manufacturer and very little sharing between manufacturers
1) how much other components are shared between manufacturers, and having companies like Delphi Automotive doing a lot of work for many manufacturers
2) how often car manufacturers own parts of each other, and how often these ownership structures change (look at Ford and Volvo and Mazda, or Daimler-Chrysler, etc)
I would imagine there are shared platforms and components, also software, between manufacturers.