
To remove the OE basic mirror, give it a sharp (not crazy!) hit in a downward motion a direction so it would slide down the windscreen.
This should pop it off the windscreen.
The wiring loom is in the headlining - you might be lucky to be able to loosen the headlining in the front area to be able to untie the wiring and pull it through.
more than likely you will have to drop the headlining.
Remove the map pockets and light which are over head - unclip wiring to lights and earth lead etc.
Remove the sun visors and clip
Remove A piller trim
Remove the front roof mounted handles
See if you can drop the headlining enough to get to the wires. You probably should be able to without going further.
Bring the wires down.
Clip in the Compass mirror onto the windscreen mount. Don't force it, it sits seemingly a fraction lower.
Plug in the mirror and test with ignition on.
Reinstall roof lining
Drive in circles to calibrate the compass.