OK, so I had a rooted nexus 5 and a VW Passat CC (5N0 035 730 B) with premium functionality.
This worked fine for several months. (and it worked for months before without android-rsap on a Samsung GS2)
Then one day, it refused to connect (it would connect and then immediately disconnect).
So, (erroneously) assuming the bluetooth module was broken (and since I'd been thinking about it anyway), I purchased and fitted a (5N0 035 730 D).
Now, my phone connects fine to the VW bluetooth system, but without rsap. ie it connects with traditional bluetooth headset profile (which the "D" revision supports).
1. re-installing the system files (and rebooting) = no change
2. upgrading rsap installation app to v2.4.6, re-installing and rebooting = no change.
What should I try next? Given the bluetooth on the phone itself seems to be working fine, I feel I have to point a finger at the rsap software.