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If you have a Uniden or Escort radar then I recommend the M1 or M2 add-on dash cam. I run a Uniden R8 w/ an M2 in my DD and then a R7 w/ an M1 in the C6. They are small and out of the way - nice little unit though admittedly overpriced.
If you're not running a detector, then I would get something like this (it's on sale today):
I had ones like this one (this size, but not this exact model) and it worked great for placing in front of the rearview mirror so it didn't block any of my view. You could also probably tap into the rear view mirror like I did for my detectors. I hate cords hanging from the window.
I have an Aukey DRO2P dash cam. If you get the optional wiring harness, you can set it up so that it will also turn itself on and start recording if it detects movement around the car when it's parked.
I have a convertible and I use the Vantrue N2 Pro Uber Dual 1080P Dash Cam.
For my scenario its a perfect choice, I mount the dash cam direct under the rear view mirror and a tad right of center and the secondary "interior camera" works just great for rear view camera usage, even better when there is no passenger in the car.