Using the wiring harness in your autodimming mirror is yet another way to power your radar detector. A-Pillar/Fuse panel route and Homelink are two other popular methods.
I prefer using Mirrortap with a Blendmount for what I think is the cleanest install possible. It's also fairly easy and straightforward. No wires need to be spliced and no panels need to be removed (other than the tiny plastic covering pieces). The one short, thin Mirrortap cable is almost completely hidden behind the rear view mirror. While I used a Blendmount, it should work with other mounting solutions equally well. If you are going to hang it from your visor or windshield you will likely need a slightly longer cable length, which Mirrortap does have available.
I used a Audi-004 Blendmount mount and 12 inch Mirrortap Microtap, which has slightly thinner (.05 vs .08 I think) pins which you will need to wedge into the Molex connector plug wiring harness. You need to find the 12v wire and ground wire in the harness. You will wedge the red mirrortap pin into the 12v and the black pin into the ground. I show it in the following photos from my own installation but if you are not sure use a multimeter.
As I indicated, I used a microtap but Mirrortap recommends a regular size Mirrortap (.08 I think ) for Audi vehicles. My Infinity m35x was a real PITA to get the pins into so I wanted the smaller microptap pins for my S7. I would stick to the regular Mirrortap size as my microtap was initially a bit loose. But I was able to snugly fit it in without a problem by wedging the pins into a different corner of the respective ground and 12v connector stalls.
See cautionary disclaimer at end of this post.
Remove plastic trim cover to expose wire harness. Slide it backward slightly so you can see the front lip detach from the middle piece of plastic trim. Then wiggle, pull, back and forth and it will eventually let go. The connector points don't appear designed to hold it too tight. At this point you can also wiggle off the smaller middle piece of plastic trim.
With ignition and power to the mirror off, detach lower wiring harness from mirror. Mine was snug and I had to use a bit of force but it came off without a problem. There is a tiny release pin which you push up on to allow the wiring harness to come free. I used the tip of one of my multimeter probes to push it in.
Step 3: Locate the 12 v and Ground wires in the freed harness. Use a multimeter to confirm the 12v and ground if you are not certain.
Step 4: Wedge the pins into the respective wire "stalls" of the aforementioned 12V and Ground wires. My wiring harness 12 V lead was the black and red wire and the ground was the brown wire next to it. I also had a second ground wire (per my multimeter testing)- the black wire third in line. So it went black/red (12v)-brown (ground)- black ( 2nd ground)- and purple wire, in that order. At this point I turned on the ignition and confirmed there was power by connecting my RD. See photo below.
Step 5: Tuck the Mirrortap cable under the middle smaller plastic trim piece such that it comes out at 12 O'Clock where the stem of the mirror attachment exits the plastic trim. This serves to not only hide the cable behind the mirror but it also anchors the slack inside the wiring harness box and doesn't require any drilling of holes or alteration of the plastic trim pieces to allow the cable to exit the wiring harness cover.
Step 6: Attach remaining larger plastic trim cover, attach your RD and plug it in, and use a plastic zip tie around the Blendmount to gather the slack in the Mirrortap cable and you are done.http://www.audizine.com/forum/images/smilies/smile.gif
Caution: I am not an electrician or a car mechanic. I am not affiliated with Mirrortap or Blendmount in any way. This is simply how I installed my Mirrortap in my 2015 Audi S7. Please see the Mirrortap instructions that come with your Mirrortap and read them carefully before attempting installation and see the warnings and disclaimers on the Mirrortap brochure/directions. If you are unsure of the Ground and 12V plug indentity, use a multimeter. I make no representation that the wiring of your rearview mirror is the same as mine. The above installation outline is simply a demonstration of how I installed my own RD with a Mirrortap. If you are still uncertain about the installation in any way have a qualified installer do the installation for you.