Wet floorboard

I'm redoing the interior on my 77 Impala, and when I took up the carpet and jute backing at the front passenger side, I noticed the floorboard was wet from about the front of the door to the firewall. The car had not been driven in at least a week, but had been driven in the rain at that time. Any ideas where the leak could be coming from? Nothing seems to stick out. Thanks!

Heater core probably

I was searching the forum and guessed as much. How do I stop the leak?

There's a cheap way and a free way
Cheap way
About 30 rockauto
And some time a few hours time to remove rh wheel and hvac housing box remove core re seal and assembly and bleed coolant
One of the members did a complete write up in how to I think it was myloth

Free way
By pass heater core
Hose from top of water pump to front intake manifold water inlet on the front right (pass)corner

I did the freeway been like that a few years now that I did my motor swap time to alot of hvac and elec work

The HVAC box is also known to leak where it meets the firewall. I've also seen where the drain on the bottom of the HVAC box gets plugged and water will run through the intake for the HVAC box and down the floor vents onto the carpet. Either way, these cars are known for floor boards rusting out for this exact reason.

If it's leaking where it meets the firewall, would bypassing the heater core still bypass the leak? Or is that leak from rainwater? Also will disassembly and heater core replacement stop the leak if it's by the firewall? I'm not opposed to doing the work just never delved into one before.

of it's leaking around the firewall, then replacing the heater core probably won't fix the problem. I would start by pulling the top off the HVAC unit and inspecting for water or damage and take it from there, since you have to pull that cover off to get to the heater core.

I'm sure you already seen this if youve been searching the forum but I can gaurantee you its the heater box. Will need to be disassembled to reseal or if you dont want to do that right now, get a tube of silicone and take the screen off the box and silicone it all up on the top part, that may fix it if its the firewall gasket, however these heater boxes are separable which means the box itself can leak if the glue inside has gotten brittle and dried up. The only real way is to of course remove the whole box. https://www.gmbbodyforum.com/t46-leaking-heater-box-disassembly?highlight=Heater+box

I agree here and it's more than likely the firewall to HVAC gasket that is leaking rain water.

I went through this entire process on my Bel Air and trust me, it's a must do fix if your car is leaking water there.

I was able to snag a whole replacement heater box assembly from a 1984 Caprice and I also grabbed the gasket as well.

I noticed that the gasket was much thicker and made from a better material then the one from from my 78, so that might be something to look into from a donor car in the wreckers.

Heater core is one location. However, don't leave out the windshield. If the top of the windshield leaks, it will run across the trim, down the a pillars and onto the floor.