Not sure but, I think the iPhone can only be paired up with one Bluetooth device at a time. For instance, I have a Bluetooth speaker I use with my iPhone. But if I'm within the vicinity of the speaker and try to pair up with another Bluetooth item, it won't pair up. You may have to go to the Bluetooth settings on your phone and "Forget This Device" (the cab radio) to be able to pair with the external unit and vice versa.
2013 Jayco Eagle 328 RLTS
2012 Ford F250 6.2L 4x4 CCSB W/Air Lift air bags
2002 Coleman Bayside Elite (traded)
Me, Momma and Charles (RIP buddy)
And introducing Sofie!! ;)