Hi and welcome.
You are right, you need to run a high level adapter, there is no line level output on the HU.
The inside of the door needs to come off, the "grill" is part of the door trim and cannot be removed seperatly.
I have read that:
"...let me tell u it was not easy trying to find a way into the car from the engine bay. but there is one, right in the middle at the back of the engine, all the electrical wires run through a rubber seal into the car."
Quote from Deemon.
There are some threads:
[HOW TO] Install a subwoofer and amp
How to install an entire ICE system
See also the Wiki:
http://www.civinfo.com/wiki/index.php?title=Subwoofers
http://www.civinfo.com/wiki/index.ph...front_speakers
Good luck!