So, quite a lot has happened actually. I sent my entire system back to Viofo UK (a second time) and they replaced the main camera, rear camera and cable. (End of Jan). This worked for approximately 2 weeks and the beeping/disconnecting camera issues returned. I updated to firmware 1.6 as well - which did not resolve any problems. We looked at the power source next. I am using the Viofo cigarette lighter plug socket (not hard wired) and tested the voltage, all was fine.
Viofo UK then offered me a swap for any other camera - but none seemed to have the same quality of video of the A139. About this same time I realised if I touched the cable going to the rear camera I could trigger the beeping. I sent a video of my entire installation and the beeping being caused by just 'touching' the cable.
Viofo UK sent out 2 brand new cables to me the last week of February. The new cable seemed like it was a higher quality and it connected to the rear camera without a normal 'click.' It was super firm and did not wiggle AT ALL. It was as if the connector was bigger and allowed for less play.
I have not had a single failure since installing the latest cable. I consider it now resolved with this new cable.
I must give props to Viofo UK for hanging in there with me and trying all sorts of things to resolve my issues. Yeah, I had to foot the return shipping multiple times - but they did send many items to me in return.