Need 2003 Dodge Durango wiring diagram

On this page they have a diagram for an 03 ram--I dunno if it's gonna be the same or not but when in doubt of course use a multimeter to verify.  Also, I'm assuming since it's a new car the factory plugs are still there?  If so, you can retain the factory wiring and still add on to it by getting both an aftermarket harness, and a reverse harness, and hooking up all the wires together (except for those you plan to tap in to), then just plug it in to the car's harness, and use the other end to plug back in to the factory radio.  I highly recommend this method as it eliminates any guesswork (all wires on harnesses are already labeled) and gives you a nice, clean reliable wiring job.

If your vehicle has steering wheel audio controls, then you're gonna need to tap in to that original factory plug--you can't use those harnesses (unless you don't mind losing your steering wheel controls!).  again, an easy way to do this is to grab yourself that same 02 and up chrysler reverse ("oem") harness and just hold it up next to the factory connector.  Since all the pinouts are the same for both connectors, and they're labeled in the reverse harness, just match up the pin/wire for whatever function you need, and note the corresponding factory wire color.

Some shops will probably let you borrow one of these connectors for a couple minutes so that you can take it out to your car and write down the wiring colors.  And then just give it back to them, and you'll have saved yourself from having to buy it.

You can get those harnesses for much cheaper elsewhere online, I just chose that particular site as it had a good picture to show you what you need to get.