The PO had installed two Lorex IR capable cameras in the ER - one oriented to the front of the engine - belts and hydraulic pump and the other looking at the aft end of the engine - shaft and coupling and bilge sump and oil filters. These have hard wired video feeds to an automotive type LCD (think DVD) display at the helm capable of two video input connections. Being IR capable they provide a picture when there is no light in the ER.
I subsequently mounted two Lorex cameras in the cockpit to give us port and starboard stern quarter views when backing down - or even seeing someone walking down the dock (side tied). The video on these is wireless to a receiver at the helm. They too are IR so color in day light and B/W at night. They came with 110v AC to 12v DC power converters. I cut those off and wired the connectors directly to a 12v breaker at the main panel.
Perhaps more details than you are seeking - the LCD display only presents a single image at a time. So with the original two ER cameras we just toggled between the two displays. The wireless receiver is capable of taking four video feeds and configuring a four image display and sending that as a single image to the LCD monitor. So I then ran the wireless receiver through a Radio Shack manual video switch (think A-B switch) along with one of the ER cameras. In this manner I can send through the A-B swith one ER camera (with the other directly connected to the LCD display) and toggle between them as before. Or I can select via the A-B switch the aft camera quad type display to be sent to the LCD monitor and then toggle to it and have both stern quarters displayed.
And finally I've purchased a third Lorex camera (found it on sale and decided to give this a try) like the two I've installed in the cockpit with plans to build a housing out of white StarBoard and mount it on the mast spreader facing aft - just looking out over the stern. This too is a wireless camera and will connect with the wireless receiver so the quad display array will then have three of the four frames filled. I will need to get 12vDC power to this. However I have some spreader lights already mounted - though am planning to replace them - so I should be able to get power to this third exterior camera from the same breaker as used for the aft cameras.
The reason for the StarBoard housing is that, while the cameras or exterior type, Lorex says they are meant to be under an eve or overhang and does not recommend mounting them directly exposed. That's not a problem at all in the cockpit - mounted under the boat deck overhang. I do notice that with temperature changes there is a bit of condensation at times on the inside of the lens cover of the cameras in the cockpit. It hasn't been a problem so far. Time will tell on this. More capable cameras with heating in the housing (eliminate condensation I guess) and pan/tilt and Internet capable along with more sophisticated monitors and recorders are all available. It's only money. I think I'll have less than $300 on all this - materials only. Of course I started with the two ER cameras and the LCD display in place.
I've purchased the cameras from both Lorex directly and from Amazon. I'd heard some reports on marginal customer service at Lorex. My experience has been okay - be patient as you may be on hold for a while. I've not had to interact much though I may need some help getting this new camera synch'd with the existing wireless receiver.
Hope this is helpful and sorry for the length.
Don