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interior water leak from passenger side under dash by heater core

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Greetings, This spring I stripped the interior out of my 81 TA to sound deaden it only to find the passenger side floor pan rusted out,UGH...... Well got a new pan and had the whole passenger side replaced. I assumed the rust out was due to leaking T-top seals as its common. We had a pretty good rain today and I put plastic under the t-top and over the glass taping the perimeter to the glass and body. Went out and water in the floor ? Started looking around under dash and the insulation behind the heater enclosure is wet and I saw a drop of water drop down. I had a guy replace the heater core some time back and I'm wondering is there some type of sealer that should be between the box and firewall or is there is something above that in cowl that I'm missing ? I checked the exit where heater core goes into engine compartment and the material around the hoses is broken so I plan to seal those, any other ideas ? Possibly somewhere I can get a diagram of this area in dash ?
Thanks in advance,

I replied to your other post. What did you want a diagram of, general dash assembly, or sealing locations? Do you have any service or assembly manuals?

Hey TA87020 - I have a 78 TA that I have been keeping mostly outside for the last year or so. I am realizing these 70-81 Fbodys leak in many places. A lot of body seams where metal meets metal were just filled with a caulking/body sealer type material. Now, 40-50 years later - these seams can often leak.
Amongst my many leaks - I had small leak near heater core. This was dripping a bit onto passenger floor. I found many of my leaks with carpet out and most of interior out. I would spray section of the body with water and check. Sometimes, I would sit in car and have helper spray to see if water was coming in. Tracing how and where the water gets in can be challenging. You could often have a leak and not even know it. As you know, our carpet insulation can hold a ton of water.
My TA has no AC (and no ttops). So, was pretty easy to pull heater core box. I recall the heater box had some type of caulking foam based ? material that was pretty worn. I used 3m caulking strips to reseal. I also replaced both door seals (large seal that has small clips that runs down side and under door).  I also replaced upper window/door seal. I used Metro seals. I took out my kick panels - this is the panel that would be to the right of your feet when sitting in passenger seat. I resealed the kick panel.  I used the 3m strips to reseal the kick panels.
So, you can see - if you have water in front floorboard area - you likely should replace heater core seal, window seal,  door seal,and kick panel seal. Really, even if these seals are not leaking now - they likely will leak. So, if you plan to keep car , not bad idea to replace. If you keep car inside and dont drive too much in rain - then whole water leaking problems - is likely not much of an issue.
I recall ready there was also a body seal where firewall meets dash ?? though for me , I dont think this is leaking (yet). I am currently working on a small trunk leak. In passenger side of trunk (forward pass trunk corner) - there is a small leak drip - water drips down (near where spare tire would be) and rolls along trunk floor and ends up at small indentation  at back of trunk near taillight. 
Apparently, there is body seam seal in this area and the seam is again filled with now old dried out caulking. I think I am going to have to lie in trunk and have helper spray and hopefully I can see where this trunk seam is leaking. I think the seam is leaking from underneath.

Like I posted, I have the 78 Pontiac Firebird sealing guide put out as an update mid-production. It shows all places used at factory and updates as well as fixes for problem areas. But no response. Can't help everyone.

Wallington - the '78 Pontiac Firebird sealing guide' sounds like it could be a great resource.
I would love to see all the places used at factory and updates as well as fixes for problem areas !
Could you post some details - maybe pics from the guide ?  It would help me to know what areas
I can focus on - when searching for water leaks.



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