The U.S. Army is equipping all of its armored vehicles with a rearview video system. Basically, one of the existing taillights is replaced with the "Check 6" unit. Looks just like the taillight it replaces, except that below the light, there is a day/night vidcam, that shows the rear view on a small display in front of the driver, or via existing displays.
The U.S. Army has already been using a similar device as part of its TUSK (Tank Urban Survival Kit) upgrades for M-1 tanks used in urban environments. Israel also has a similar system on its tanks, and some of its infantry fighting vehicles. Many commercial vehicles are getting the system as well. But in combat, having a constant view of what's behind you can be a matter of life or death (a tank is most vulnerable to attack from the rear).
Currently, if a tank crew wants to know what's behind it, someone has to open a hatch, and risking getting their head shot off, to take a look. Each "Check 6" system costs a few hundred dollars, and does not require special tools.