It should still work (engine bay terminals) even if the negative battery post is NOT attached or even if the positive terminal is not attached to the battery. If they attach to the
dedicated negative post in the engine bay (instead of selecting a bolt on the strut tower or other metal piece that they
think is grounded…which may not)….then the vehicle is still grounded if 12v is running though the engine bay terminals.
What could stop the show on the f10 is if the 12 volt connection that runs to the electronic boxes is some how disconnected. You can see in the pic in my post above that there are approx 5-6 connections on the 12 volt side. I’m not sure which ones carry power to the e-boxes…but like in the pic below…as long as the BST is intact, power continues to the fuses even when the positive terminal is removed from the battery.
You can see an example in this simpler setup found in the e53 x5 positive battery cable run from the engine bay to the battery onto the electronics box. Again, if power is applied to the terminals in the engine bay…as long as the battery safety terminal is intact…12 volts will continue on to the electronics…because the negative terminal in the engine bay will provide the ground.
