Could be any number of things but my first thought is the external socket mounted on your car. Is it clean and dry? any sign of corrosion on the terminals? These sockets are a weak link due to exposure. It may not be the cause in your case, but it's worth fitting a rubber cap to protect the socket when not in use (also on the van socket). I found the smallest size rubber chair leg cups from Bunnings fit perfectly on the SD sockets.

I had a similar problem when the monitor suddeny stopped working I found that I had damaged the small pins because I had probably not not lined it up exactly when I previously connected the monitor at the dash. I rang Safety Dave and he sent me a new cable section which included the new male connection. When I explained to Safety Dave what I had done he sent me the new part and didn't even bother charging me for postage - it was a 10 minute job for the auto electrician to replace the section of cable from inside the front door pillar. When I had my mine wired up I paid extra to have the curly cord and the weather proof connections - not cheap but I reckon it was worth it.

Cheers
BB

Screen isnt powering up