I just did the same thing in my truck. I had an Infinity system with just am/fm/cassette and swapped in a am/fm/cassete/cd from a Dakota sport. I didn't change any speakers, just the radio.
You have two screws for the cup holder, two screws for the ash tray thing, you can pull the dash. The dash pops right out but you have to lower your tilt steering all the way and disconnect the power outlet.
Now you can pull out your radio after removing the two screws that hold that, unplug the antenna, two plugs for the radio stuff, and one ground wire.
Reverse to put it all back together.
It took me about 7 minutes from start to finish (not counting getting tools together).