For those with fully hidden cables (i.e. passing inside the stem / headset) I would suggest putting the bluetooth junction somewhere in the stem or just behind the bars to make it easier to disconnect the entire handlebar setup in one go.
I put mine just inside the downtube so I can disconnect the bars easily if I want to. Not that I can, given hydro hoses are a bit harder to disconnect.
The bluetooth thing in the bars seems pointless but as above; it's how I've got my bar-end junction bike set up.