I install rear view systems quite often and I would suggest something like the following .
http://search.ebay.com/search/search...+lcd&category0=

or ...

http://search.ebay.com/search/search...n+dash+3.5+lcd

The LCD rearview mirror replacement is what I am going to use . All power and signal is through one small din cable that goes up to the headliner and down the door post so there is really nothing to see . You can get Bluetooth also which displays your calls on the same screen and has a built in mic/spkr for hands free calls . The whole thing is a regular rearview mirror until the video signal is applied and then you see the screen .

I've seen many problems from the cheap wireless units plus the displays usually do not have good enough contrast/brightness to be visible in many daylight conditions or large enough area (field of view) to be of much use .