Other Car Rental Programs/Partners (ie. Alamo, Enterprise, Sixt) - Sixt Germany CPMR with GPS

I am looking to book a CPMR (BMW 1er or Volvo C30) for a week long, oneway rental between Friedrichshafen West and STR. When I try to book on the website and add 'guaranteed' GPS, it changes the vehicle type to a VW Golf. Is there anyway to get a CPMR with GPS that is still the BMW or Volvo?

Also, does anyone have a Sixt discount code that can be used by the general public?

TIA.

There is no way to get CPMR guaranteed with GPS (I really do not know why). However, everytime I booked "GPS guaranteed" I got a BMW1 or an Audi A3. So just mention that you would like to have a BMW1 while making the reservation and it should work...