DA said SW cleaned car thoroughly in and out after the murder. So who knows. Is it easier to clean blood off leather seats than cloth? I'm still curious about the rental car.
http://losangeles.cbslocal.com/2018/02/02/blaze-bernstein-murder-suspect-court/
There is a substance called luminol that causes blood to fluoresce. It's used to identify trace blood left behind after a thorough cleaning. Surely, the rental car can be (or has been) identified and checked, as well as SW's vehicle. I suppose once trace blood is found, it can be collected and tested for DNA.
Have we established that it was his family vehicle that he cleaned (as opposed to the rental)?