How to unclog a drain

[smkcbe]

I have a 2004 530i E60. Recently, I found a lot of water in the passenger side rear seat footwell after raining. I have been reading the forums at length and everyone seem to point to drains in the sunroof, the door, and the fresh air intake being clogged. I have tested these drains and they are all working properly. I went as far as taking the passenger seat out and lifting up the carpet. I finally traced the incoming water to the firewall where the wire bundle from the battery goes through the firewall on the passenger side. I then proceeded to investigate the problem under the hood. Once I removed the fresh air intake cowell, I saw a pool of water just under a large plastic box which looks like it contains wire connectors for the battery wire bundle. I believe there is suppose to be a drain hole at the bottom of this area, but I don't know how to access it. How can I clean out a drain hole when I can even get to it? I'm hoping someone here with more knowledge can help me clear this drain out and stop the water from coming in again. Thank you in advance.