Scott
This is the dashcam I bought from Amazon:
E-Prance New Mini 0801 Original Ambarella A2S60
It's small enough to fit behind the mirror, although I did notice you mentioned you have sensors there. Assuming the fuses are in the same location (drivers footwell) on the D3, I ran the cable along the headlining, down the A pillar and around the dash. I then used the following two items to connect it to the cigarette lighter fuse.
12v Female Car Cigarette Lighter Socket
Piggyback Fuse
The excess cabling and Socket were then tied up underneath well above the bonnet release catch. I was a bit concerned before ordering that the socket fit might be loose, but it's a really snug fit, and there's no chance of the plug falling out.
Apart from the chime when it starts and stops (with the ignition), you don't notice it at all. The only visible sign is if driving at night, when the small blue flashing LED reflects off the back of the mirror, but a small bit of tape would cure that.
This is what it looks like from outside:
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds