My wife got me this back up camera a couple of years ago. It is from Wal Mart and is supposed to be wired to the back up light. I have it on whenever the car is on. It helps with the poor rear visibility on my 72 Mach 1.
That's a good idea and a good location inside for the monitor. My son has an in-dash Video/Radio in his 2001 Mustang GT Bullitt that has a connection for a backup camera. I was going to get him the camera for the rear of the car for Christmas, but I forgot about it, maybe for his birthday. Ditto on posting up a picture of the camera on the rear of your Mach.
I have this on my list of mods myself. There is a JVC EL Kameleon kd-avx44 that I believe will fit in the existing slot. The cool thing, I think it is anyway, about the system is that it has no knobs on it - it goes completely black when not in use and when in use you can tilt it up. The stereo slot on my 72 is low so this is a really good feature that you can tilt it up look down on the rear image. There is another system that ends in -77 I am going to look at soon, I am not sure if this is the next generation or something dramatically different in features but I do know you can upload an image to the faceplate. The rear camera jumped into my head almost immeditely when I saw the rear clearance. Another location I was thinking was thru one of the honey combs. I will definitely be doing this I don't know all of the hows yet or when but when I do I will take a picture inside and out.