I haven't found a wiring diagram either!
I do have a parts book (pdf) that has some Wiring Harness diagrams, I determined that there were some other things that share the Earth, see a few posts further up.
If it's an Earth issue they should not be working, If they are working then it's something else and I've reached the limit of my knowledge.
It's worrying that your issue started like mine is at the moment,
I use a Torque Pro App and Dongle from Amazon and don't have a problem connecting but it didn't show any faults. Interesting that the Diagnostic port is on the same Earth
Good Luck
