Hardwire radar detector installation?

I'll assume you are not in DC or Virginia where they are illegal to operate. Based on that, first install the detector where you want it in the vehicle. Then look at the trim around that spot. Many times, you can tuck the wires behind that trim using a flat dull tool. I would route the wires to the driver's side and down to the fuse panel located under the dash on the driver's side. Now you need what is known as a fuse tap. It plugs into the fuse box in place of a fuse, then you plug two fuses into it. The original fuse and a new one. Now you will find fuse number 18 and install the fuse tap in its place and the fuse and the new fuse into the tap. The positive wire from the power lead then gets connected to the wire on the fuse tap using either soldering and heat shrink or with a crimp connector with sealant. The ground wire then gets a ring terminal crimped to it and attached to a convenient metal ground area.
Installed that way the power would turn on when the ignition is on and off when it's off, so you don't have a dead battery.

https://www.2carpros.com/articles/how-a-car-fuse-works