Water leading into car between the firewall and carpet. Both sides get wet when it rains, water appears to be between the firewall and the firewall carpet as the firewall carpet does not get wet but the floors do

There could be some clogging of the drainways below the hood, resulting in water entering the passenger compartment. Also check if the AC drain is clogged leading to water entry into the passenger compartment.

Had similar on 02 Highlander. Source was leak from around window trim (rust) and roof racks connector entering and going down the windshield post behind the trim. Gently remove interior plastic trim piece to the left of the windshield to investigate any water coming down behind there. Find source, disassemble whatever is required and use something like SupraExpert to seal (not just cheap silicone).

If its cold during your rainy weather and you need heat to warm the interior of your car. You might want to have your cooling system pressure checked. The heater core might have developed some corrosion which will eventually lead to leaks. If you have radiator coolant in your cooling system, touch the wet carpet then rub your fingers together. There should be a film left on your fingers. Hope this helps.

Sounds like it's the heater core

Have you been adding any coolant? You could have a leaking heater core, which will usually leak to the inside, wetting the carpets in the well. Most American cars locate the heater core on the passenger side of the firewall, so a leak will typically show up on only the passenger side floor. I once had an Audi that had the heater core mounted just about dead center on the firewall. When it started leaking, both sides of the well(s) got wet.

A 99% bet is that the heater coil is leaking. It is located in the dash on the firewall-passenger side.

If you have a wet seat then it is likely defective weather stripping around the window.

Try this. If the floor is dry, then it rains and the floor remains dry, only gets wet after a drive in the rain or on wet streets, you may have a hole somewhere.

Check for holes from missing rubber grommets or holes in the floorboard or firewall from rust. Check for any rust holes or missing rubber grommets under the car or on the firewall, anywhere water can splash up into the vehicle from driving in rain or running through puddles..

Check wet spots along the carpeting all the way up under the dash, if the wet spot is higher than the floor itself, you could be getting the water from splashing through puddles.

Any holes under the car or in the fenderwell even up on the firewall can cause this problem.

First check for any obvious signs of rust, if none are found then look for missing or defective rubber grommets.

I think that I can help you. It sounds like your drain-tube is plugged up. Get in the passenger side, and pull the carpet back from under the glove box. Look for a black rubber pipe that goes from a black box, and pokes thru the firewall, on the passenger side. Remove the hose from the box, and have a container ready to catch water. If water comes out of the black box, your drain hose is plugged up. Clean out the clog, and you will be golden.

Have your ac evaporator deain cleaned out or checked. Without this working the ac water will collect there.