We haven't tested any mirror dash cams simply because each one we've looked into requires the rear camera to sit outside the vehicle - which means getting the rear light cluster professionally removed so the wiring can trail through it. Our advice for a front and rear dash cam would be either the Nextbase 512GWRC or the Z-Edge T4 dual dash cam, both of which we've tested and which we have linked to below. You can find footage in our reviews as well as the cheapest places to buy it currently. In terms of using a rear camera as a reverse camera, that simply won't work. The rear cam footage is shown in the corner of the dash cam, making it completely unsuitable for use as a parking camera. I'd recommend getting a proper parking camera or using something like the FenSens (review here: kit.honestjohn.co.uk/reviews/review-fensens-wirele.../)

Nextbase dual dash cam review: kit.honestjohn.co.uk/reviews/review-nextbase-512gw.../

Z-Edge T4 review: kit.honestjohn.co.uk/reviews/review-z-edge-t4-dual.../