It is not uncommon for people with some DIY skill to splice into wires at the mirror, but the hard wire kits have wires long enough to go from a fuse box usually down low and up the A pillar to the middle of the windscreen.
A few cars might have the battery or fuse box out back, in that case you would have to extend wires, and doing so on the 12 V side of things are also fine, extending USB cables, at least the generic power one to the camera wire can be a problem, for sure you want to go with a quality cable and not cheap CCA cables. ( Cobber Clad Aluminum )
There are a few cameras that actually have 12 to the camera, most often using a small 2.1 or 3 mm barrel plug, these most often then have the low voltage cut off for parking guard build into the camera itself instead of the 12 -> 5 V conversion box in the 5V based systems.
I think both Thinkware and Blackvue do systems with a 12 V input on some models at least and probably also a few other brands., then you could shorten the power wire down a lot and insert a small inline 5A fuse.
You can also modify, many cameras have the 12 -> 5V converter in the 12 V plug itself, but the PCB in there are actually pretty small, so you could liberate that from the bulky plastic housing, solder a +12V and 12 GND wire on ( with a fuse on the +12 V of course) , and then hide the PCB in a piece of shrink tube, or as i did years back with a NAV unit just wrap a lot of electrical tape around it.
Street guardian have the 12 -> 5V converter in a box a few feet up the 12 V cable above the plug, this mean you can snip off the 12 V plug and use fuse adapters to tap into a fuse if you want to keep the dash socket free, this free option however do not give you parking guard, just a nice option to have to keep the dash socket free.
You can also shorten the normally rather lengthy 5V cable on dashcam power sources, it should just be the 2 wires in there +5V and 5V GND, you of course then have to solder things together and insulate that with heat shrink tube to make sure you dont short things out.
Fairly simple if you have a little DIY skills and a soldering station.
There always seem to be a lot of surplus wires working with dashcams, some times i have it bundled up under the dash / in front of the glove box, or i have shoved surplus wire in over the headliner to be "stored " there.
As i have been in the game for many years i have made some modifications to my car for me to be able to accommodate a lot of systems, some times i will have 4-5-6 systems in the car with up to 12 cameras in total.
So in my glove box i have 2 X 3 12 V sockets, and also 2 multi port USB chargers that can also run a dashcam if need be.
And i am also tapped into a few fuses now as i have to be that in order to test parking guard features in systems.
My 12 V socket in the dash are free for other things.
In general the trim plastic on A pillars are easy to pry off, and most often all you have behind it is a guide strap for the curtain airbags, and some of the cars own wires.
I need new trim plastics as by now they have been off / on so many times they can hardly stray in place, my place is to go to a salvage yard this summer and find something i can use.
Routing a rear camera out back, you have to be carefull with the wire and curtain airbags, the wire need to be above or behind the airbag so it is not in the way if it deploy, you can maybe also route cable down low if the car are not too big, or you can go in under the doors weather strip where the wire can most often fit in under the rubber.
it is also a good idea to first do a dry run of the rear camera cable loosely draped along the intended route, cuz the rear cameras have been known to interfere with DAB and SAT radio, so if routing in one side is no good, maybe the other side is better, and more easy to find out before you have spent a hour routing the wire out back.
For this test just shove cable in along the headliner and screw curtain airbags, it is just a quick test in the driveway / garage.
