Newbie here. So I own a 2007 G35X and I live in Hawaii where it's been raining pretty hard in the recent days. I realized a couple of weeks ago while driving in the rain that my left foot was oddly wet. So I look down at the parking break pedal and there was water dripping and running down the dead pedal (the place your left foot rests while driving). I kind of panicked a little at the time but I didn't think too much of it - until a couple of days ago.
It was raining hard for about two or three days straight, carpet completely wet, and my car was starting to smell pretty bad. So I decided to lift the carpet up to see what was going on under there, and I found about two or three inches of water. I vacuumed the water out and got a little over half a gallon. Realizing that this is really bad, I tried locating the origin of the leak, and I found that the leak is coming from above the parking brake pedal - the joint were the parking brake assembly attaches to the bottom of the dash with bolts.
Reading forum posts here, I was able to come with three potential causes: sunroof drain leak, windshield seal, and grommet seal (the one by the brake booster).
So I tested the sunroof drain after a day of rain. I poured a small amount of water in the gutters on the drivers side. Water was dripping at a faster rate. However, I believe that it is unrelated because today I poured a total of 2 liters in the gutter and the water seems to be draining fine under my car.
As for the grommet by the brake booster, I haven't tested it yet, but it doesn't seem as if thats the problem. I saw a small trickle going down to it, but the trickle goes around the grommet and goes to the bottom of the car (so it doesn't seem as the water is entering the grommet).
Today I hosed my car for about 20 minutes in different places at different angles in attempt to try isolating the issue with no luck.
If anyone has any clue with where the leak could be, it would be great help. I don't think I can afford going to the dealer and paying whatever they charge me to get the problem fixed, as Im only a college student and my income isn't really that high at all. So it would be great if I could resolve the problem on my own at a low cost.
Ill see if I can get some pictures of where the leak is coming from etc., if it helps.

 

I had the same problem and it was the big wire loom behind the brake booster. The sealing grommet/plug just wasn't doing it's job anymore. Go to home depot and get some electrican's conduit putty or get some permatex rtv and make a new seal around it. The electrician's putty is removable so you could always start with that and keep trying other locations.

 

Thanks for the reply. I was initially suspecting the grommet to the right of the brake booster and after taking a look at it, it seemed like it was well stuck onto the firewall. I went on and spent half an hour sealing it off all around with weatherproof silicone caulk anyways. I thought the problem was fixed but after another two days of rain, I peeled the carpet back again and found a little less than half an inch of water. I'm starting to suspect that it's the area where the sunroof drain hose attaches to the firewall. I haven't been able to test it yet but I was able to see the hose link to the firewall by removing the drivers A/C vent. I'll see if I can get any further tonight. I hope that's the problem but at the same time I don't want that to be the problem since I'd have to take the entire dashboard off to reach the hose mount on the firewall.

 

UPDATE: I finally figured out the source of the leak. Turned out to be the part where the sunroof drain hose attaches to the wall between the cabin and engine. It's right behind the dashboard. I was disappointed when I learned that I'd most likely have to take apart the entire dashboard to reach the part, but I was able to seal it with caulk through the a/c vent nearest to the drivers door. I just took out the vent and attached two chopsticks together with tape to make it long, and I slowly dabbed caulk around the plastic grommet with the tip of the chopstick.