Sunroofs/convertibles =water world..over a period of time ,Ive found.
Guaranteeing no water ingress with sunroof/convertible in the long term ,would be like preventing Katie Price from writing..no sorry(Im not even sure she can write)..I mean from publishing more books with her name on them.
Megane 3(my one) and I think with some other posters ,even with a complete hard top:
The water will come in with blocked scuttle/windscreen drain holes.
Even when my windscreen drainholes are clear ,I can hear a steady drip drip on both sides,if Im in the car and the engine/radio are off.
Hopefully its going down behind the front wheelarch and down to the ground,but you can see other meg II and 3 posters with underfloor storage boxes filling up wi water.
If you got a car under guarantee,you can moan and get them to strip interior ,dry out everything and get it solved..Ive seen cars being dried out when Ive been in for servicing.
If its up to you.. look for water stains on interior trim,check roof drains.. etc etc