Volvo+Jaguar=Ford
Hertz offers those luxury vehicles because I believe just about their entire fleet is Ford vehicles. Similarly Avis uses almost entirely GM vehicles therefore their premium vehicles come in the form of Cadillacs. I think all the major rental car companies use American vehicles (I think theres some incentive) and maybe an asian car here and there. My understanding (though I hope I'm wrong) was the only way to get a BMW or Mercedes (or Audi) as a rental is a. through a dealership or b. those expensive luxury rental places. There might be some smaller somewhat localized rental car companies that offer German vehicles, just depends on where you are I guess.