Don't know for sure since I own 87 El Camino. My 87 is a 5th Gen and I believe similar/in the same generation (5th) of G bodies to which yours belongs.
Here are a few suggestions and thoughts for you to consider:
(1) Can you tell if the fluid is plain water or possibly anti-freeze? You will be able to detect a smell if it is anti-freeze. If it turns out to be anti-freeze, then the source is a heater core leak.
(2) To verify a clogged drain, I suggest you first check the bottom of the AC box in your engine compartment. If neither you nor any previous owners have done any work on rebuilding the box, that is possibly a likely source of outside water dripping into the cabin. Unfortunately, the flap over the drain is underneath and along the firewall; it is not readily visible from the top. You may be able to feel the flap and drain area from under the car.
However, the best way to do a visual check is from on top of the AC box. The G body has a screened plastic piece (3-4" x 8-10") that allows outside air to enter the cabin; the screen is visible and accessible next to the base of the windshield from the engine compartment. The screen is held in place by several screws and can be easily removed. Once removed, peer down to the bottom of the cavity and you'll likely see a lot of debris ranging from twigs, leaves, dirt, and grime that has collected over the years. This view will also give you a good idea of where your drain is at the bottom of the AC box.
Good luck!