Join Date: Feb 2010
Location: Newport Beach, CA and Melbourne, Aus.
Are you sure the phone is transmitting at full strength? My iPhone 6 has developed the same problem in my '14 Merc and turns out this is a well documented problem with them. To the point that iFixit sells a kit to repair them.
Although now I'm confused because of your topic title says Bluetooth but you talk about cables.
1973 105 GTV (Alfa #6 of 19 owned)