2003 dodge,output on stock stereo?

You'll have to use a line output converter because the only wires that you can hook up to are the speaker-level wires in the radio's harness.  The line output converter is used to adapt this amplified signal into a cleaner RCA type signal that can be used for an aftermarket amplifier's low-level signal inputs.  Any good 2-channel converter will work just fine for your situation-- PAC, soundgate, and David Navone all make good LOC's.

Usually people hook up to the 4 (+ and - for each of the 2 speakers) speaker wires going to the rear speakers, but the front ones will usually work as well.  All you have to do is look at these diagrams ( #1 or #2) to figure out which of the wires in your radio's harness will need to be tapped in to.  Assuming you're tapping in to the rear speaker leads, you'll want the "RR+", "RR-", "RL+", and "RL-" ones.

Once you've figured that out, just strip off an inch or so of insulation from these 4 speaker wires and solder the converter's input wires onto the newly exposed section of wire in the radio's harness.  Make sure that you're just tapping in to the wire without cutting the existing connection, because if you cut the connection you'll lose sound going to the corresponding factory speaker.

yes that's correct.  The line output converter will not cause a noticeable change in impedance.

This page will give you the basics about LOC's: http://www.eatel.net/~amptech/elecdisc/loc.htm