During a ride, make sure your Bluetooth headphones are on and able to pair. Then, tap the headphone icon on screen. When successfully paired, the icon will change from white to red.
If this does not work, double-check that your headphones are not connected to any device. To confirm this, on previously connected devices, tap "forget" in your Bluetooth settings on your device to forget the Bluetooth headphones then try to pair again during a ride on equipment with a mounted tablet.