It works for me. I just downloaded the latest nightly and copied my own mycar15.xml into the routing folder.
I started OsmAnd dev and created a new profile, where you first have to choose one of the standard profiles, in this case Car.
I saved the profile, reopened it and under navigation settings now selected my mycar15.xml profile.
It does not work immediately for some reason. I don't know why, and I don't know either whether it previously worked when clicking "Create new profile" and immediately being able to select the new one.
Also when creating another new profile I can not directly select my mycar15.xml, but only the existing profiles, where my previously created profile "auto 1.5" is one of them.
See attached if you want to test mine and compare it with yours.
Feel free to share yours so I can have a look at it.
Harry