Revived

What could be the issue? My assumption is a leak when it rains but I'm not sure where to check. Anyone familiar with this issue in any way?

ES300NZ

is it in the front or the back ? what year is your car ?

if it's in the back i would check the boot, and spare tyre well for excess water caused by water flowing over the edging of the boot lid (do you park on an angle?)

if it's at the front it could be quite a few things but a good start is:

are the door full of water ? (open and 'shake back and forth')
is there rust around the windscreen ?
below the front windscreen, is that full for leaves?

Revived

It's the back.

The boot?

WILLYumD

Boot is a common alternative for "trunk".

Do check the spare tire as well as your sunroof drains.
Wouldn't hurt to check if the rubber lining/gasket of your trunk is clear or not.

ES300NZ

If it's in the back, and the car is parked on an angle, the top corners of the 'trunk' can fill up and overflow, over the rubber, into the boot, and then down under the rear seat and out onto the carpet.

shortstop

Mine was the Sunroof drains, if its on the floor boards. The sunroof drains get clogged up, the channels get overfilled with a heavy rainfall, and spill down the inside of the roof pillar, and sit in the floor boards.
Clean them out with compressed air, then pour some water in the sunroof channels to confirm water drainage from the side drain hoses underneath the car.
When fixed you will have to remove carpet and insulation from the wet area, to fully dry everything out to prevent mold. All sunroofs are designed to leak a little but some will need new seals after years of use.