Having a camera on the outside of the car are not advised, the optimal will be to have them on the inside and if possible in a place covered by a wiper.
I am fortunate to have that both front and rear on my little JAP car that could fit in the bed of that pickup
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@Dashmellow in here are probably the go to source for multi camera systems in trucks.
Tapping into the fuse box are pretty easy too ( lots of videos on youtube on how to do that, which are almost a no tool needed job ) i did that and installed a 3 X 12 V adapter in the boot of my car, and it even have a 2 A USB plug too.
Took +12V from the fuse box ( seat heaters ) with a piggyback fuse adapter and grabbed the -12V from one of the bolts holding my rear seat in place.
You can also get multi port USB power supplies/chargers, i have a couple of those in my glove box to power /charge everything USB related ( though i will not use a long USB wire to power something in the back, hence the 12 V expansion i made back there.
Those USB chargers you would want them to be over 2 A on each port, and it require a little careful shopping as what is claimed isent always true.