there is couple of fuse you can tap into under the glove box.

 

Is this mounted on a GS? Wouldn't the rear sun shade hit the rear camera when it's in the "up" position?

 

The 4GS's motorized rear sunshade will only allow a very slim dashcam to fit between the glass rear windscreen and the rear sunshade.
Models that fit include the StreetGuardian SGZC12RC, the Viofo A119 Series, and the forthcoming slim StreetGuardian SG9663DC as an example.

 

The BlackVue rear camera will also fit. I have one installed on mine.

 

The top of the rear sun shade touches the rear camera - but just barely. Definitely not enough to cause it to detach - I've had it almost 2 years now, and I haven't had to touch it yet (despite using the rear sunshade very often)

 

In the Blackvue DRS-650 Series twin channel dashcams, the rear camera certainly does fit in between the 4GS rear windscreen and the motorized rear sunshade, however do remember that the 650 Series rear dash cam is only standard definition at 1024 x 768 pixels, and not high definition at 1920 x 1080 like their front dashcam.
Furthermore, the 650 Series rear dashcam runs at a low bit rate, because a single CPU must process data from 2 channels simultaneously, the bit rate from each channel must be reduced, unless a bigger, faster processor, requiring much more active fan or liquid cooling is used - hence the low bit rate compromizes the video quality.

Below is a video comparing a Blackvue DRS-650 single channel front camera to a StreetGuardian SG9665GC in low light; the StreetGuardian kills the Blackvue already.
If the StreetGuardian is compared to the rear of the dual channel Blackvue DRS-650 Series, it will kill the Blackvue further.