Non-Yaris Audio install help needed

[Revsson]

Sorry if this is in the wrong section, but I need some audio installation advice.

Over the years, I have installed radios and speakers in my cars and for some
friends and relatives. Nothing fancy, just an aftermarket radio and
sometimes an amp and sub. So, I am not a totally noob to wiring, but I've
come across an install that I need help with.

Before buying my Yaris, I had an amp and sub in my old car (Geo Metro).
I took out the amp and sub when I sold it, and now my niece wants them
both. She has a 2004 Dodge Stratus with a factory radio that has a CD
player. They want to keep the factory radio and not install an aftermarket,
as she likes the current setup, but wants a little more bass.

Here is one problem: The car is about a 2.5 hour drive away, so I can't
just peek under the dash at the wiring. I searched online, and could only
find a wiring listing and no schematics. Not sure if this is from installer.com
or crutchfield.com, but here is what I found:

2004 Dodge Stratus Car Stereo Radio Wiring Diagram

Car Radio Constant 12v+ Wire: Pink
Car Radio Switched 12v+ Wire: Red/White
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Orange/Brown
Car Stereo Dimmer Wire: N/A
Car Stereo Antenna Trigger: N/A
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Front Speakers Size: N/A
Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): Dark Green
Left Front Speaker Negative Wire (-): Brown/Red
Right Front Speaker Positive Wire (+): Violet
Right Front Speaker Negative Wire (-): Dark Blue/Red
Rear Speakers Size: N/A
Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): Brown/Yellow
Left Rear Speaker Negative Wire (-): Brown/Light Blue
Right Rear Speaker Positive Wire (+): Dark Blue/White
Right Rear Speaker Negative Wire (-): Dark Blue/Orange

Notice that there is no wire for me to hook up to the remote wire on the amp.
Where am I going to get the signal for the remote wire?

Second problem: Adding a volume pot to turn down the sub when necessary.
My brother had someone install an amp and sub in his 2002 Grand Am GT to
the factory stereo. They put in a volume pot so that if there are police
around or just want to have less bass, they can turn it down. He sent me
a pic, and it appears that they are using the RCA inputs on his amp. I'm not
sure if they are taking the speaker outputs and going to the pot and then
the output of the pot going to the RCA inputs on the amp. There is no pic
of the wiring going to the pot, as it's installed in the dash somewhere.
He would like me to install a similar pot when I put the sub in my niece's car.

The amp I will be using is an Eclipse that has what they call the high level
input (from speakers) and also has low level inputs (RCA). I know I can
buy a wiring harness, so that I don't need to splice into the factory speaker
wires. However, if I try and wire up the pot like his is, then I should probably
take say the LR and RR speaker outputs and run that to a stereo pot?
Then take the output of that pot and go to the speaker input or RCA input
on the amp? Or is there some sort of adapter (cheaply I hope) that would
do the same thing I am trying to do? If I am taking part of the speaker
output to feed the rear speakers and then split off to feed the amp, won't
the impedance be thrown off by adding a pot?

Of course, it would all be easier if they would have taken the Eclipse CD radio
that I took out of the Geo and not have that extra pot in there. I'd like to
be able to have most, if not all of the parts and wires in hand so that when
I drive down there some weekend, I can install it with little or no trouble.

Any advice from the audio gurus on this forum?