[astepabove]

unfortunately I think its a common gran turismo issue .water flows down the rear window/trunk drip rails and through the inside of the rear bumper (next time you wash your car if its working correctly you'll see two puddles directly in the middle underneath the car. If theres too much water going down at the same time / perfect angle (a rain storm as the perfect example) the water wont go straight down those channels and instead starts to pool in the area directly under that cushion in your picture because the actual interior trunk is plastic bolted on to the rear steel car and isnt sealed with a gasket or atleast the gasket isnt great... as soon as you take off that cushion theres 3 /4 torx bolts that I imagine are rusty since thats where the water is coming in from. (a bunch of us here have the same issue and mine is a 2015 so you woulda thought they fixed this simple defect...)

 

[astepabove]

I have seen those torx bolts and you are correct. Are a bit rusty. Do you think the water comes in from the bolts? In that case I can use a sealing o-ring or something like that? Or I need to unscrew the whole thing and put something like a gasket underneath (if the water comes in between the 2 parts)

Yes it's absolutely coming in from the plastic trunk not sealing with the rear steel of the back of the trunk (can't tell you if its due to flexing of the materials/ no gasket between the two or the Torx bolts loosen over time and cause the no seal ) when you take off that padding you have in your picture that's where the water starts pooling and then if there's too much water or you drive around / at angles then it will eventually make its way to the bottom of the trunk since that's the lowest part there's multiple other members who have commented or documented not taken off that trunkliner ever or not replaced battery in years and then when they finally do they see that antenna/motor sitting in the plastic trunk is all rusted out too.

 

[astepabove]

You are correct. I removed the cushion and the water comes in the trunk from there. The question is why the water gets blocked underneath the torx bolts. Even If somehow we seal the two pieces then, I could assume that the water will still be blocked somewhere else in a different area of the car. How we can make sure the water will go down to the floor.

realistically if you want to fix it permanently its going to require you to completely remove the plastic box and clean out the entire area underneath and then use a silicione gasket material and maybe some rubberlines to completely block it. It would seem to be a engineering defect from maserati that wasnt intended or they just didnt care to fix lol as body of the car is basically the same from 2007-2018 so the issue theoretically should affect any car that sees significant rain.I would not recommend putting the cushion back in that spot as its going to just get moldy and cover the water.

Since the MC version has a different exhaust layout that comes out the middle of the rear bumper I wonder if the trunk box is shaped alittle different which would also make the water drainage different, but knowing how they made this car with super cheap materials in some places im guessing they made as little changes to the models as possible for cos.

. I just double check my trunk every time theres been a major rain for a week or so just to make sure nothing is wet in there as my trunk is usually empty anyway so it only takes 5 seconds to pull off the carpet top and smuggler box and inspect and clean with a towel.