I've been getting water or moisture in the passenger footwell which leads to a moldy or mildew like smell.

I'm planning to pull the carpet at the very least and probably replace it. Any suggestions on where the leak may be coming from? Any experience with this?

 

clogged or missing evaporator drain tube

 

The car doesn't have A/C (it's a 92 Civic Sedan).

It doesn't necessarily happen after a heavy rain, it just seems like it appears on humid days especially if I leave the windows closed. It's not a puddle, but the floor in the front passenger side is wet to the touch.

 

*DOh! Well if it is just random "moisture" I'm not sure. Do you have stray cats around? maybe they are using your ride as a pee hole.

Check the heater core/hoses.

Look at firewall from engine bay and you'll see two heater hoses going through firewall and into the vehicle. Check heater core. If leaking, replace it. Or, disconnect those two lines from firewall and splice them together with connector to bypass heater.

 

touch the carpet, lick your finger, is it coolant? if so, time for a new heater core as stated above

 

^ If I do that, I'll probably get some disease!

I'll take a look at those lines for the heater core. Would it still leak if the heat hasn't been used? This has been happening for some time now including through the summer.

Another thing I was thinking it could be is maybe water leaking from the windshield, but again it's done this when it hasn't rained and has just been humid out too.