First off, the mirror mount on any Nissan is the same as the standard Gentex mirror mount so you don't need an adapter.
Once you unclip the plastic trim on the original mirror, you can push the mirror upwards to remove, leaving a small metal mirror mount stuck to the windscreen. To attach the Gentex mirror, slide the new mirror over the mirror mount and tighten screw to fasten into position.
Route cables along front edge of headlining. There is no need to do anything with the headlining. There is a gap where it meets the top of the windscreen. Just push the cable into the gap. Route the cable to the passenger side. Pull off internal trim down windscreen pillar in order to route cables down side of windscreen. Route cables under front of dashboard, connecting nagative cable to suitable eathing point and positive cable to red cable on auxiliary socket wiring.
If like me you get a mirror with tempeature sensor, the hard part of the install is to route cable to front of car, but you can get a cable through main grommit in bulkhead behind battery and attach temperature sensor to lower front grill.