1) If you got the whole muiscarNW kit he supplys a really long ground and
power and the intention it to have it go right to the battery tightening bolt on
the battery clamps.
This is suppose to be the least noisy connection point in the car.
See my install procedure in the HI Fi section of
2) I thing you are going to have remove the side trim . I did for the Jehnerts
and you will have remove more for routing your harness.
http://www.e90post.com/forums/showthread.php?t=641323
I think things should look like what I did with the wiring running out to the sides from the TCU area and over the wheel wells and the amp in the same location.
I didn't have to route wire under the carpet so I can't help you there .
The rear seat bench comes of pretty easy it you just pull it up. Grab it on the
foward edge and pull up. (it takes a pretty good yank )
Take pictures of your wire routing if you can.
Also be aware that you can drain your battery pretty easily when installing because almost anything you do around the car keeps it awake.
Someone was mumbling something about a shop mode for the car that
stopped that but I don't know how it works.
I tried to do my work in stages and check it and test it but that is a little
easier to do with a hi fi. amp first test , door speakers test , floor speakers
test , if I had any problems it would at least be isolated to that section and
is less of an overwhelming task.