On my 2006, the factory plug was under the drivers side dash, near the emergency brake. I purchased a "Dodge to controller" cable (plugs on both ends, prewired) for 12 bucks at my local dealer (like this one
http://tinyurl.com/2xqaka ). Pluged it into my controller and then the factory plug and wham, all done.
As stated above, you only have that plug if your Dodge has the factory tow package.
Here is a nice walk thru if you have to wire the truck yourself.
http://tinyurl.com/yuftqv
Hope this helps.
Jim