Best method to eliminate alternator whine from a 12V line?
Three issues. First - the isolation in the DC2DC should be OK to maintain an RF earth through the system. There is a slight doubt that if it is too good then it may impair the antenna performance. Second - the DC2DC is only 1 watt rated so is this going to be enough - probably enough for the BT receiver but don't plan on any other devices that might take  more than 1 watt.

Another thought is that due to inevitable leakage capacitance inside the DC2DC, it might not solve the  original problem. You've got to consider that although it is isolating (because it uses a transformer) the inter winding capacitance (which provides an RF earth) might also propagate the original problem.

Having said all that it's a cheap enough unit to try and throw away if you are not happy with it.

One more thing about the DC2DC - I don't think it is a regulating type - if you look at the datasheet on page 1 the 12 volt version only has an operating input voltage range of 10.8V to 13.2V and if your lighter socket can produce 16 volts there's going to be a problem. I think you should consider a better device.

If your BT receiver only requires 5V I'd consider a linear voltage regulator but choose one that is peak rated at up to 60 volts because now and then these are the sort of input voltage spikes you can get on a car wiring system. Same applies to any regulator type including any DC2DC.

A linear voltage reg will probably do a better job getting rid of the whine if it isn't earth related but imposed across + and - wires.