Motor Shield and HC-06 Bluetooth Module competing for pins
The motor shield doesn't actually use the TX and RX pins. Though as a shield it will go into all the arduino pins. 

You could either solder some wires to the TX and RX pins, or use something like the proto shield to sandwich in the middle of it, and connect the BT module onto this shield.

You could also remove all the male headers from the motor shield, and replace them with stacking headers