Looking more at the docs, I recommend trying to change this line 972 from:

const String btPinCodeString = "SET BT AUTH * 0000";

to

const String btPinCodeString = "SET BT AUTH * 12345678";

where "12345678" is the correct 8-digit pin code for your car stereo.

Does that work and allow it to pair?
_________________________
Tony Fabris