Where to run power cable for hidden amp

I would think you want the "signal" wire interconnected with either the radio deck or whatever feeds the amp, not necessarily the key. You may or may not want the radio fed from the key as it was stock, depending on preference

Don't you have a shop manual / wiring diagram?

What year? mode? is your car?

Shop manuals: Free, by the way:

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

More stuff here

http://www.mymopar.com/index.php?pid=31