Someone was referencing this how-to in another thread and I thought I would clear things up for the record. You do not need to run your power cable over to the passenger side. In fact I will go so far to say you should NOT do it because that means you will most likely run your RCAs down the driver's side right past the fuse box which has a tendency to induce noise into your system. Furthermore, if you run your power cable down the driver's side you save 3-4' of cable.
Here is what you should do instead:
http://www.msprotege.com/forum/attachment.php?attachmentid=19373
In the above picture, you are looking at the firewall behind the battery. The grommet you want to go through is highlighted. You can pop this out from under the driver's side without removing any panels. Pop the grommet through from the engine bay and use a flashlight to find it on the other side. Carefully cut a hole in the rubber with a small knife (I used my swiss army knife), slide your cable through, and pop it back in.