Thans for the response always appreciated.
I spent half the night trying to figure this out. It looks like the cheaper units use RCA and accorded to the internet the RCA should not be used outside. Although shrink wrap should make the connection water tight.
The better units use 4 pin aviation connectors and also carry the power for the camera. CAn buy RCA with a power wire also. Then they sell an adapter for 4 pin to RCA.
I was happy I figured it out and all ready to order and found there are AHD cameras and analog cameras, the head unit needs to be compatible with the camera. Analog will give the marking lines for parking and the AHD does not. So another evening spent figuring out which is which before ordering.