2013 w204 with Comand ntg4.5 aftermarket reverse camera only displays for a few seconds then goes black

[alanp10]

I have just installed an aftermarket reverse camera into my 2013 C250cdi and have a really good clear image for a few seconds then goes black. Any ideas would be appreciated. I wired the camera into the reverse light by splicing into the positive wire and the earth into the nearby fuse box earth.