Bosch bought Denso shares as an investment back in the 50s? That seems weird. Does Ford own millions in Chrysler stock? Home Depot in Lowes?
It's not unheard of. Did you know that Ford owns Mazda and Renault owns Nissan? When you drive a Mazda3, you're riding in a Japanese car with European Ford Focus underpinnings. Ditto with the Mazda2, with Ford Fiesta underpinnings.
It's part of globalization and economies of scale.
As far as MB, that would explain the use of certain Denso components, such as the air pump, A/C compressor, etc. I take comfort in havng Denso when it comes to the A/C compressor as Germans don't live in a climate that dictates A/C use as evidenced by the Euro MBs where A/C is optional.