Do these Transit connects have a fusilbe link on the battery earth strap like some of the full fat Transits?
And you are only getting a weak earth through accessories on the battery!!
Only a thought because I had one once when the driver of the vehicle tried to start his van in gear and blew the fusible link and was only left with a poor earth through his Amber flashing beacon lights

Easy test, just put an earth jumper cable on battery earth and connect to body earth
Ian