K12 Rear Seat is drenched help?

My Micra rear seat is soaking after sitting parked in the rain for days.
There appears to be drips and condensation build up around the High Level rear brake light (so would be dripping/leaking directly onto boot floor?

Has anybody else experienced this? Appears to be a line of black silicon along the top of the tailgate glass which is gone at the passenger side.

Is the tailgate glass meant to be sealed like this? I will post a picture shortly (its raining at the moment picture wouldn't be that clear).

How easy is it to remove the rear glass and reseal and refit?

Note no signs of drips, or water running down the interior of the car so i am fairly confident this is the cause.

Many thanks

Report Edit
Back seats not been wet before but wheelwell once had a pudle in.
I looked about and thought high up on boot lid or rear door.
I do have 3 door so rear 5 doors won't apply..
The side window has black silicone looks original.
I plan to remove both side as I'm replacing with a metal sheet eventually.
But like to know if the is much lip for me to do them. Possibly do pop out style using the glass like in k11s..
As this is something missing that we're a smart idea.

Water getting in tho usual spots are sunroof door seals and window seals.
The rear door sometimes the seal can fold over might let a bit in.
Rear lights can be re sealed sometimes the seal gets cracked or shrinks.
The rear door handle is also a good place to check that's what I was getting ingress from it's been fine ever since.

Last thing to consider is possible condensation since in UK it's pretty much raining most of the time.

My rear boot lid was under a dripping gutter.

Best thing is to check through and make sure every possible area is sealed properly from elements.