Nexus 7 Bluetooth does not pair with phone automatically

[mowienay]

I use my Nexus 7 (2013, Android 6.0) as a car mount. It has been useful for navigation and multimedia. To have internet connectivity, I pair my tablet with my phone (iPhone or Blackberry Classic) via bluetooth.
I am facing an insignificant but tedious problem. When I enter my car, the Nexus does not pair automatically with my phone. I have to go to bluetooth settings and pair manually. I am wondering if this is by-design or if there is a way to make it pair automatically.
On a side note, is there anyway to flag a specific bluetooth connection as cellular/mobile data, the same way we can flag wifi hotspots?

P.S. I am not using Wifi hotspot to reduce energy consumption of both phone and tablet, and mainly because the poor iPhone sends the wifi-hotspot to sleep, if inactive for a while.

 

[B. Diddy]

Welcome to Android Central! Is the phone also paired with the car's own media system, or with a Bluetooth headset? I'm just wondering something else might be competing for the Bluetooth connection.

 

[mowienay]

Thank you for your welcome message !!
Yes, the phone is connected to a bluetooth fm transmitter. Yet, when I later pair with the phone from my Nexus settings, the phone is paired to both fm transmitter and Nexus simultaneously.

 

[B. Diddy]

Just to test, try forgetting the FM transmitter on the phone, and see if the phone starts connecting automatically with the tablet.

 

[mowienay]

Ok, that is a good point. Will try it tonight !!