Taping the front dome light/map light is what I did for the homelink mirror I installed in my '14 SV to provide unswitched power. Since the S and SV come with a auto-dimming mirror there is already a switched lead to the mirror pigtail.
If you replace the auto-dimming mirror with a Homelink auto-dimming mirror, you can feed both the auto-dim circuit and the homelink circuit with the existing switched line. If you want the homelink circuit to be alive all of the time, you can take a tap from one of the map lights to feed that. That is what I did.
Two things to consider.
No need to rig a switched power connection on the S and SV, that line is already provided to the standard auto-dim mirror.
If you upgrade from an auto-dim to a homelink/auto-dim mirror, the pigtail that connects to the mirror is different between the 2 types of mirrors. However the correct type of pigtail is readily available on eBay