I have the Garmin Dashcam 55 fitted on my A3 8V, it is located behind the left side of the rear view mirror. I initially had it on the passenger side behind the sun visor, but I found that when the sun visor is in the lowered position it tends to hit the dash cam, especially if the passenger is heavy handed.
Cable is tucked into the roof lining, and down the left hand side A pillar trim towards the fuse box located behind the glovebox. Not hard to hard wire into the car for a neat and tidy outcome.
It is advisable to upgrade the 8GB microSD card to at least a 64GB card, you will find 8GB tends to be overwritten very quickly with daily driving. Update the firmware via Garmin Connect before installation, change the video setting for 1440 30fps recording (stock setting is 1080p) as well as disable the voice command option.