9 Feet will not be enough, 20 feet is a bit too much, but the excess I just tuck under the front passenger seat. It creates zero problems like that. Furthermore, instead of running from the USB port inside the armrest (which will most likely not give enough USB voltage), I ran from the 9v outlet behind the armrest towards the floor of the rear seats. I picked up a 2.4a charger like this (
http://amzn.to/2iMJ6U1) which comes with two 2.4a USB ports and one 3.0a USB port for quick charging a phone. Then just route the cable under the passenger seat, across to the lower door frame on the passenger side, up the side of door frame near the glove box, and then you can just tuck it into the A-pillar molding. Once you reach the roof it can be tucked entirely into the roof headliner. It's a mostly clean install that's very easy to undo quickly since the cable is just being tucked into place without needing to remove any parts. You do see a tiny bit of the cable as you route it from the door frame to the A-pillar, but that's it. The only other way I'd do it is a more permanent solution of wiring it straight into the fuse box behind the glove box.
BTW, the reason that I installed it in the rear 9v jack is because that port stays on for 10 minute after the car is turned off. That actually keeps your camera running for the full 10 minutes, sort of acting as a temporary parking camera. That method actually saved me $900 thus far because it caught some asshole back into my car and leave the scene 3 minutes after I had parked my car at Epcot. The front 9v jack works differently, and immediately turns off at the same time as the car.
I'm actually now running two dashcams also. Using a similar method I wired in a dashcam to the rear, also powered from that rear 9v jack. Routing the wire was slightly more tricky because I had to figure out a way to mount it on the rear of the trunk. But I ended up just routing some extra cord in a way that it gave a bit of extra slack that tightened up as the trunk raises. Hard to explain, but it's pretty clean and totally invisible unless the trunk is opened.