I've been searching around for awhile to see if anyone on the internet has done a hardwired connection for their dash cams to their rear view because I hate running extra long cables and wanted something easily removable. I could not find any. So here is a tutorial of how to do it on a 2017 Maserati Ghibli SQ4 (with driver assistance).
Materials (yours may differ depending on what cable you need):
Electrical tape (or heat-shrink if you have it)
Total cost: ~$6 per car (I did mine and my wife’s and still have 3 extra boards...)
1) Buy all the parts if you don't already have it.
2) Remove the mirror cover. (I do not pictures of how to do this, so proceed at your own risk. Hint: the upper part pops out on either sides and clips hold the rest of that piece in. The main body piece then pops downwards)
3) Identify the power source from the rear view mirror harness. I have already done this for you. (your's may or may not be different since I have a 2017 SQ4 for Driver Assist).
4) Put together your Buck Converter by soldering the Red jumper wire to Positive and Black wire to Negative and attach the USB cable. (I have attached a picture that shows the terminal points to solder)
5) Wrap up OR Heat shrink OR whatever method you want to protect the electronics and your wires. (I used electrical tape for this time and zip-tied the wires to keep them together)
6) Insert the jumper wires into the harness aligning the corresponding colors.
7) Secure the module in place by zip-tying the module to the wire harness.
8) Plug in the other end of the USB cable to your device
9) Reattach the mirror cover pieces.
10) Start the car and enjoy your clean setup.
Like I said before, this can easily be undone by just removing the mirror cover and pulling the jumper cables out. I hope this helps all of you out there that are looking for a better way to hard wire your 5V accessories to the rear-view mirror.