I would buy a plastic auto trim tool set on Amazon and start by prying out the square plastic plug near the top of the tan trim where it meets the head liner. I would bet there is a bolt hidden under there. After the bolt comes out, there are trim clips to be popped off. They can sometimes be temperamental and fragile so take your time. I haven't taken apart a XC90, my experience is with a V70 and V60, but Volvos tend to have similar assembly methods.

 

Here is a link for the factory tow hitch installation. From step 30 or so, it shows how to remove the right side panels and some more. Should help you figure out the pieces you need to remove.

https://accessories.volvocars.com/en-us/InstallationInstruction/Pdf/VCC-513804-2/XC90(16-)/2018/T6%20AWD/Automatic/default/XC90%20(16-)%202018%20America

 

Oh wow, that's excellent, just what I needed, thank you very much!

 

Thanks for all your help guys. I discovered the leak actually wasn't coming from the sunroof, rather it seemed to be coming from the tailgate hinges. I put some silicon around both and now can no longer replicate the leak. Fingers crossed!