Minimum ive found you need to take off to access pipes easily is:
Roof Console where sunroof handle + light is etc
Sun visors
Roof Handles
A pillar trim (just give it a careful pull towards you if your sat in the front seat)
B pillar trim, need to take off bolt holding seat belt.
Possibly the top part of the door seals, as they can partially cover the headlining
Once all that is off, if you open your sunroof and look on the inside part near the edge of the glass, you should see 2 screws at the front and 3 each on the side, these need undoing, then the headlining should drop easily and you can access the pipes easily.
A simple check before you do all this though, is wind open the sunroof and pour a liquid of your choice down the inside channel of the sunroof, if it doesnt disappear, you know which side is blocked.
If it happens to be the passenger side thats blocked, a potential very quick fix is loosen the bolt thats right next to the bonnet release catch. It holds up part of the carpet and goes through a dash holding point. The bolt can intercept the drainage pipe and cut it off, resulting in a block, loosening it can be a quick fix, but might still need to blow down the tube to clear it.
I put some spacers on my bolt and put it back on, so its secure but doesnt go in far enough to hit the pipe.
If its not that, or its the drivers side, hot water, unblocker stuff and an air compressor are your friends
Of course, could just be the aerial, if it is, silicone it up.
Be on the look out as well incase any standing water in the sunroof channels has started to rust through. If it has, your best off removing the sunroof entirely (torx screws underneath where headlining was) and treating it, because itll only get worse.
Re fit is, of course, reverse of the above.
Oh, also, dont undo the two torx screws that attach the winding mechanism (that the handle sits on) to the actual sunroof. I entered into a world of annoyance with that one
Sorry if post is a bit long, theres also one or two picture guides floating about