Assuming you have an automatic transmission, there is a hole in the firewall on the driver's side, and it is on the firewall and up fairly high, so it is relatively accessible.
I left the factory plastic blanking cover/plate intact, and just drilled a hole into it using a right-angle drill and one of those conical stepping drill bits so I could get the desired hole diameter (don't recall it offhand) without using a hole saw or changing the bit a bunch of times.
I placed a rubber firewall boot in the hole, knowing this might not be the last cable I need to pass through the firewall and into the cab:
https://www.amazon.com/gp/product/B0051ZG5K8/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
I brought my (switched - I will have to go check the fusebox to see which circuit I ended up using) 12v lead through the boot and into the cab. The 12v lead is soldered to one of those "add a circuit" fuse taps for ease of installation and security/safety of the connection:
https://www.amazon.com/gp/product/B00OZGA8O8/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
I will not make an automotive electrical connection without using a soldered joint, and when I am finished with the connection, I wrap in electrical tape first, then heat-shrink over the top just to be sure it is waterproof, etc. YMMV.
With the power lead inside the vehicle, I added my own fuse block and stowed it underneath the cupholders, just ahead of the center console in that fairly spacious area to hide connections and gear:
https://www.amazon.com/gp/product/B000THQ0CQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 (again, with extra capacity - thinking about future connections)
I then ran a blendmount hardtap power cable from the detector, tucking it into the headliner, down the A pillar and under the dash, routing it to the center console fuse block I installed:
https://www.blendmount.com/mirrortap.html (I could have tapped power at the mirror, but chose not to because of the complexity of the connection and the limited area to work in there - I have had varying degrees of success with the mirror tap connectors, depending largely on the specific vehicle, type of mirror, etc).
In the engine bay, I covered all new wires I ran with PowerBraid:
https://www.amazon.com/gp/product/B000N8IR2W/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
This gives a clean, almost OEM look to the wiring. Using it with zip ties and Tesa Tape:
https://www.amazon.com/gp/product/B00EH6IZ6Y/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
really keeps it neat.
I hope this was helpful. Let me know if you have other questions.
-Rob