Need help with water leak

Hey guys the other day I went out to get something out of my truck after it had stopped raining and I noticed that the driver side carpet was pretty wet which I thought was weird because I hadn't driven the truck in a while. So the next day another storm was coming so I grabbed a towel and stuffed it up in the corner between where are the wiring comes down and the brake pedal. After the rain stopped I went out to check carpet to see if it was wet but it was dry. The towel on the other hand was soaking wet so now I found there are where it is leaking but have no idea where the water is coming in. I checked the firewall from inside the engine bay and couldn't see any holes so now I am stumped. Where do you guys think this leak could be coming from?

Here are some pics of what it looks like where the leak is coming from

 

I had a leak where the wires come through in the firewall close to the brake booster under the hood. Water would come right down the channel

 

How did you seal it?

 

Well mine happened after running large power cable for my stereo amp....I used silicone. Never had water down there again.. I kinda tore up that big rubber grommet that the wires went through

 

+1 on that same thing same grommet

 

I'll check that grommet but I haven't run cables through the firewall