Rear floor pan plug is separating

[4drPwr]

I have been getting an awful wet spot in my rear passenger floor since owning my car, usually after heavy precipitation. I could never track down the cause, but from the rear seat wall forward to about 6" or so shy of the floor vent under the front passenger seat, I would lift up the carpet, the foam backing would be SOAKED, carpet wet, floor mat back damp, and the metal floor pan would be obviously moist at this point.

Today I was cleaning my interior and since we had heavy rain yesterday, decided to check the carpeting for water. It was dry, but I did find that the tar stuff on the floor was chipping badly and falling apart. I picked off a few big chunks, and found that the circular "plug" is partially separating and if I were to lift it up, I could see through to the pavement beneath. My driver's side rear floor is completely different looking and has never leaked. Did someone modify one side of the floor? How can I reattach the "plug" securely and permanently to prevent the elements from getting in? What is the putty-looking stuff on my driver's side rear and can I buy/use that? Thanks for any help

You can see the differences:

Passenger rear (you can see where I pulled off the tar in the center, this is where the plug is separated):


Driver rear:

[4drPwr]

Bump for help?

[thereisnoyun]

passenger side is normal, the black tar like stuff is the sound deadener use back in the 80s and 90s.

the driver side someone must have put what look like chaulking. I think the piece are snap in place, and the bottom is coated with under coating which kind of grips it, and the top is sealed the way it is.

What I can tell you is that its not hte hole on the ground cuasing the leak, its from the rear. If its a cab it is 100% likely to be the plug ontop of hte shock tower, if its a coupe it is likely the sunroof drain.

[4drPwr]

Is the sunroof drain suspect for a sedan as well? What can I use to seal it like on the driver's side? It's pretty firm, not like normal caulking. I'll be pulling the carpet to install dynamat on the entire floor later this summer so removing the tar is not a concern to me.