A growing thread on the Android issue tracker is home to more and more reports of in-vehicle Bluetooth problems with the Nexus 5, 6, 5X, and 6P. The N5 and N6 allegedly only became affected when their Marshmallow updates rolled out, while the 5X and 6P, which run Marshmallow out of the box, have allegedly had problems from day one. Notably, the Bluetooth issue being cited supposedly was not present in the “M” developer preview releases for the Nexus 5 or 6, suggesting something changed recently to cause it.
[Bug Watch] Vehicle Bluetooth Call Audio Problems Being Reported On All Four Nexus Phones With Marshmallow was written by the awesome team at Android Police.
Source: Andriod Police