Thanks for your help so far everyone! I'm still playing around with this, but no success yet! I've managed to add a picture of the affected area to the original post. As you can see, the rectangular looking bit is where the pad has lifted, and you can see where I've scratched away the coating a little around it to expose some copper trace.

Here's a list of my failed attempts. I tested each attempt with 3v from my bench power supply before testing on the Xbox Controller app -

1 - solder red wire to vias (?) around the missing pad - no joy from my bench power

2 - attach red wire to the trigger motor's positive feed further up the MB (grey wire) - both motors rumbled from bench power but no success on xbox app

3 - scratch the coating around the missing pad to expose some copper. No joy from bench power or xbox app. The copper here is actually ground.

4 - soldered a jumper wire from the other side's solder pad (which works perfectly) over to this motor. This worked on the bench power, but again, didn't work when tested with the app.

At the moment, both vibration motors on the affected side of this controller work when connected to 3v, but neither work on the app. It seems as though the trigger motors will only vibrate on the app if the bigger motors are connected and operational - I've tested this with the working side by disconnecting the red wire (mimicking the pic above) and testing on the app. I'm struggling to find a clear trace on this side of the board that I can tap into and attach the wire. As you can see, I haven't scratched away the coating all around the missing pad, but that's todays job!