I had a BlackVue DR590 2-CH in my 2018 Silverado 1500 but wanted a different dash cam that had an inside view so I decided on the Garmin Tandem, but did not like the mount to the windshield. I wanted to mount from my rear view mirror, but afaik they do not offer that option so I had to piece a few parts together. I hardwired it and used F22 (heated steering wheel) for accessory and F3 (garage door opener) for battery using pigtail fuse. I would like to use F23 MISC R/C for accessory but not sure if it would work. It does not have power until accessory/run but unsure if the dash cam would draw/leech any power so chime in if I can use it.
To mount to the rear view you need to buy two dash cam mounting kits. One to mount to the rear view stalk, and another to source the ball that mounts to the Garmin because where you pop the ball in it is smaller than any kits I could find. I just routed the cable up above through the headliner and down the pillar (I did not remove) and around the passenger dash down towards the passenger fuse panel. It was very easy to do it that way, instead of pulling the trim off. You only see a short section of wire on the dash, but it is barely noticeable.
Here is the kit that allows you to mount the dash cam to the rear view mount:
https://amzn.to/3xZiaXP
Here is the rear view stalk mount kit:
https://amzn.to/3w2igfL
Garmin Tandem dash cam:
https://buy.garmin.com/en-US/US/p/671095
Parking mode/hard wire kit:
https://smile.amazon.com/dp/B06XTBPYS2?psc=1&ref=ppx_yo2_dt_b_product_details