Using a Nextbase 512 and hardwiring kit ( essentially a piggyback fuseholder and low voltage limiter), but similar for most cameras.
Remove the driver side dash end cover - small screwdriver / knife etc. Then pull off.
Remove the driver side dash fuse cover - pulls off.
Locate the required dashcam position, mine is behind and under the rear view mirror, as far up as possible. I am using a suction base for now, then an adhesive base when happy with the position.
Clean the mounting position.
Plug the connector into the camera.
Run the supply wire carefully behind the Eyesight equipment at the top of the screen, ensuring it is well clear of the cameras etc.
Push the supply wire behind the headlining on the drivers side where it touches the windscreen and down to the side trim on the driver side "A" pillar.
Tuck it behind the top of the side trim along to the door seal.
Pull the door seal away slightly and tuck the supply wire behind it all the way down to the end of the dashboard. The door seal can be pulled away completely on that part of the door, it goes back easily.
Locate a fuse to supply the camera - I used No. 7, a 15 amp one, it supplies the 12volt sockets. See 12 - 10 in the owners manual, it is the third from the left on the second row down. I used this one because I wanted my camera to turn on and off with the ignition.
Using the fuse removal tool from the engine bay fuse box I removed the fuse.
The Nextbase hardwiring kit uses a piggyback fuse - push the removed fuse into the spare location on the piggyback, there will already be a 2A fuse for the camera in place.
The existing Subaru fuses are very short and have plastic between the fuse blades, this has to be trimmed back to fit or a suitable replacement fuse obtained ( Halfords HFS 215 is a small pack of various fuses). A replacement longer fuse is best.
Plug the Peggy back fuse arrangement into the fuse box and ensure the fuse holder is located correctly and nothing catches.
Plug the other end of the piggyback connector wire onto he camera lead - I secured mine with a bit of tape as well.
On the end of the dash, above the bonnet lever is a cross point screw holding the dash trim to the metalwork - remove this and add the earth connector and wire from the hard wiring kit. You may need to cut off the existing connector and crimp a slightly bigger one on. Ensure a good connection to the metalwork of the car to complete the circuit.
Turn on the dashcam and then the ignition, the dashcam should operate.
Turn off the ignition only, the dashcam should go off after a short delay.
Cool all the excess wire and tape it up into a tight neat bundle and push it up at the side of the fuse box. Secure in place.
Replace the plastic dashboard end cap.
Replace the fuse remover and cover in the engine compartment.
Set the dashcam to point in the required direction and go testing!
You may need to use a ferrite core if interference is found.
so far after two days everything seems to work ok.