There’s nothing really to enable.
Go into your bluetooth menu, touch the settings “gear” icon beside the connected device, and there is a check box that says “Internet Access”. Hit that button and it will hook up PAN.
But one thing to keep in mind, is that Android does not “remember” that setting, so it has to be re-enabled every time the bluetooth connects. Its easy enough to write a receiver to catch a bluetooth connection event and add a PAN connect. It should also be possible to implement such functionality in the CAR HAL for those working with automotive overlays.