I've seen people install back up cameras integrated with Nav system on an older vehicles, used to cost $500 - $700 for professional installation. I'm not capable of doing that but some gifted folks should be able to.
I did it before myself on my wife's '05 Prius.
The Prius had a MID unit+Display w/o backup camera.
The installation difficulty level was medium.
A switch box was used that split video signals (R/G/B) from MID unit and backup camera (also R/G/B). The trigger wire was the reverse gear signal that needed to be pulled from somewhere (vehicle dependent). The output R/G/B wires were fed into the Display (i.e. two sets of R/G/B wires as input, and one set of R/G/B wires as output, using reverse signal wire as the trigger). That was pretty much it (other than power connection, grounding, etc.)
Installation of ultrasound sensors is much easier. I did that twice for backup.
For front, the tricky part is where you pull trigger signals from....
I would imagine that, you probably want to trigger it when...
1. speed is less than 5mph (VSS wire and compute pulse count per second)
2. foot on brake (brake light signal)
Need a device to AND these two signals together and generate an output trigger signal.
(maybe such a device can be found online... not sure)
Feed that signal to the unit of ultrasound.
That said, the ultrasound sensors are not that precise. Within a foot of distance, it beeps constantly....