I'm going to do my mirror with my stereo hopefully this weekend. I just wanted to add an alternate way I found for the power wire(s).
There is a product called "Add-A-Circuit". I think the main manufacterer is
Littlefuse but I found a similar product at my local electronics supply shop. It's basically an adapter that plugs into a fuse slot in your fuse box and gives you 2 fuse holder slots. One for the one you just pulled out and one to power a new accessory. The extra fused slot has a power lead with a crimp connector on it. There are pics on the above link but I coulnd't copy then since they are on a PDF.
Edit found one:
I'll come back here and let you guys know how it worked out.