I own a TomTom Go 500 SatNav that automatically pairs with my Samsung Galaxy Note 3 over a Bluetooth for live traffic updates. For this to work, you must have Bluetooth and Bluetooth tethering options enabled on the phone.
Bluetooth is normally not a problem - once enabled it will stay ON until you disable it yourself.
However, Bluetooth Tethering option doesn't stick so well. If the phone is shut down or rebooted, Bluetooth tethering always goes to a disabled state. Tethering also gets disabled if you toggle Bluetooth switch or Airplane mode.
One, partial solution to the problem is a free app called Bluetooth Auto Tethering ensures that Bluetooth Tethering is automatically enabled after every start-up or reboot. This app doesn't enable Bluetooth Tethering after toggling Bluetooth or Airplane mode switch. To get this bit fixed I used an app called Tasker (£2.99), which can use "Bluetooth Auto Tethering" as a plugin. Simply create a profile that enables Bluetooth Tethering every time when Bluetooth status changes to "On".
If "Bluetooth Auto Tethering" doesn't work for you (it was crashing for me on some devices), you can use Secure Settings, which also integrates with the "Tasker" in a similar way.