Backup camera in mirror - source of power?

I'm installing a backup camera in my '16 CX-5 Sport this weekend - one that is embedded in the rear-view mirror (Auto-Vox brand). There are a few great write-ups here on how to do so - thank you very much for those.

I have one quick question though - it seems like when people need power up by the mirror area they are going all the way down to the fuse panel for that power. However the OEM 'Homelink' mirror hooks into a harness that is running up the A pillar already. Why don't people just tap that wiring and draw the power from that point?

I'm assuming the Homelink mirror doesn't actually draw that much power, whereas my video-screen mirror will - which means that I should go down to the fuse panel and draw power directly for my device, but I want to double check - seems like it would be easier to get the power from the A-pillar area.

Thanks!

Not sure about getting power from the HomeLink harness, but very interested in a seeing a write-up/review of your project. I have a '16 Sport Manual also and the camera would help greatly.