Water leaking into down A pillar into drivers floor pan

Check the cabin air intake. Chances are the weather strip that lines part of the opening has come off. The intake should be accessible beneath the trim at the base of the windshield on the passenger side.

Water is probably coming from behind the dashboard. It will be from the windshield or the firewall that water is getting in.

That depends upon the cause of the water. There are a few possibilities, depending upon the vehicle. See this list and use what applies as a way to narrow down the source.

1. Rust hole in floorboard. Only way to fix is to repair hole.
2. Leaking door seal on passenger side. Locate and repair leaking seal.
3. Leaking sun/moon roof. Some vehicles that have these run their drain lines down the passenger side door pillar and down thru the floor pan. Sometimes the hose shrinks with age or breaks and allow water to get on to the floor. Remove the passenger side kick panel and inspect and repair as needed.
4. Leaking heater core. Feel and smell the water. If it smells like antifreeze or has an oily feel, it may be coolant leaking from the heater core. Of course there should be a loss of coolant associated with this problem. Here the only solution is to bypass or replace the heater core.
5. Leaking windshield seal. Most car windshields today are glued into place. These glue joints can break loose and allow water to leak. Also, rust can form around the windshield seal and cause rust holes that can leak. Lastly, extremely high-powered car stereo systems that pump mega-bass can cause overpressure inside the vehicle that can cause the seal of the glass to separate.

You don't way what type of vehicle you are driving. I will assume it has a sun or moon roof. Most of these have drain lines that run down inside the driver's side door pillar and exit through the floor pan onto the ground. Over time, these drain hoses dry out and shrink, often retracting into the drivers side kick panel. Then, all the rain water that drains from the sunroof will leak out onto the drivers side floor. If that is the case, you will need to remove the drivers side kick panel and repair or replace the drain line and reroute it through the exit hole in the floor pan.

Get your heater core checked. May be leaking.

Are you certain it is rain water? It could be condensation from the A/C coil that fails to drain out of the drain pan, or possibly coolant from a leaking heater core. If you are certain it is rain water, depending upon the age of the vehicle, and possibly some other things, here are the possible options:

1. Leaking door gasket.
2. Leaking windshield seal due to rust
3. Leaking windshield seal due to improper installation
4. Leaking windshield seal due to acoustic (sound) over pressure (if you have extreme sound system with heavy bass)
5. If you have a sunroof, if the drain hose comes down the passenger side door column, it could be leaking into the passenger compartment rather than drain through the floor pan.

Most water leaks come from around the windshield area. Has the windshield been replaces recently? If so it may not be sealed properly. Another area where water may penetrate is through the cowling area under the wiper blades/arm where fresh air enters the car. There are water drains under this cowling allowing water to drain and if they are plugged with leaves/debris, water will enter the vehicle. Is the vehicle rusted around the windshield area? If so, suspect this to be the source of water intrusion. Most glass/windshield repair companies will diagnose water intrusion problems

is it only when the ac is on? there is a pan under the ac unit that has a hole in it to allow the condensation to empty. if that hole gets plugged it could cause the condensation to build up and back into the inside of that car. i unplugged mine and floor stayed dry

There is a great possibilty that there is a leak on the Left Side of the Windshield. It is most likely in the area of the upper left side.
The reason for this conclusion is:
The rain water is coming in the Left Upper Side; and either coming down the "A" Pillar, or traveling underneath the trim on the inside roof and coming down the "B" Pillar.
The only other explanantion is a torn, worn, cracked or dislocated Weather Seal on one or both of the doors, allowing rain water to come in. Inspect very carefully these Weather Seals, especially on the top front portion of the rear door.
To help you locate these after a hard rain:
Take the trim panels off the "A: and "B" Pillars and the Roof Line above both Doors. Check for wetness underneath the trim. Or take these trim panels off, and place folded paper towels under the trim panels; reattach the trim panels and wait until it rains. Afterwhich, take these trim panel off and check the paper towels for wetness.
The Windshield Seal is easily inspected for cracks and tears.
Let me know if this helped, or if you have additional information or questions. Feel free to contact me at FixYa.com!

I don't think it has anything to do with the sunroof. I think you have a leak either around the windshield or under the cowl where the wipers are attached. If it's in the cowl area probably over the blower motor housing on the right and the water is following the vent system to the drivers side. Please let me know how you made out.

Thank you