I am not an expert, nor have I done an installation on the Pacifica. However, I have run into an issue installing a stereo into a car with a factory amp, which I think you have. Essentially you either have to use an interface
like this, or wire around the factory amp with a "bypass" harness thus driving the speakers with the head unit amp and/or additional amps you install. You will see the link is from Crutchfield. They are great to work with and their database is good for research. Click on car audio and then click on "outfit my car". That is how I found the link I posted. Good luck with it.