So after this last rainstorm, I had some water that pooled under the carpet in the rear, driver's side footwell.
I removed and exposed everything around the seat and used a hose to test different areas of the car.
The only way I can duplicate the leak it to run the hose down the driver's side windshield.
The water seems to empty nicely into the cowl drain and out under the car.
But eventually water starts to seep out from under the driver's seat rail area.
So either the drain is leaking somewhere or there is a leak around the drain area.
Has anyone had any experience with anything like this. I'm guessing somehow water is getting into the body of the car and finding its way back.
The car is parked on a slight angle - front higher - so that would make sense I guess.

 

At the age of these cars, one should consider that every body seam provides a potential leak path. The assemblies are mostly sheet steel panels that are overlapped and resistance spot welded together. On the assemblies that are meant to also provide a barrier to water intrusion (battery tray, cowl area, sunroof channel etc.), the seams are sealed with a brushed or sprayed compound to prevent water from travelling between layers and through to the other side. The seam "sealer" applied in 1980 something has long outlived its life expectancy and should be inspected and reapplied.
In your case, I would start by removing the door opening sill trim to verify if the flow of water is along the wiring path (common) and if coming from the front or the rear. The water test suggests the leak is likely coming from the front where there are more possible entry points. Considerations in addition to body seams are the penetrations in the door jamb for the wiring harness and door switch and if an early car, the antenna cable that enters through the side of the footwell.

 

I just did a quick google search:

 

One should be careful with the weed wacker line as the sunroof drain tubes are fragile at this point. Also, you cannot clear the right rear drain this way due to the wye fitting which joins the fuel filler compartment drain tube.

 

The attached TSB on possible water entry points should help.


Tom
'87 951

 

That TSB is great, thank you, but for rear hatch leaks it didn’t include the leaks I found: The rear hatch attach points have a well with a drain tube. My drain tubes work fine, but I found that when water collects on the spoiler, the drain slots drain too slow and water seeps between the spoiler and the hatch and in between the rear hatch rubber seal on the underside of the glass hatch. I peeled back the underside hatch seal around the latch pin area and added automotive rubber-gasket-in-tube. I found this out when I sat in the rear hatch and had my wife put hose water on top of the spoiler. I could see immediately water seeping and dripping from around where the hatch pin is on the glass hatch. So far, so good.