I have installed 4 different dash cams on my cars. To best answer your question really depend on how good you are with splicing wires. If you google "subaru rear view mirror wiring diagram" You will find which wires you need to splice to the dash cam. If you don't care about parking mode, then your only concern are the Power and Ground wires. If you want to use parking mode, well...then it is probably best to wires them all the way to the fuse box, since you will need to have constant power wire connected as well.
Personally, I recommend to just run the wires down to your fuse-box using tap-a-fuse. The wires are easy to hide and route and you avoid having to cut/splice wires to OEM wiring harness.
Hope that helps.