1) buy an 04+ dash bezel, with the double din opening
2) remove stock headunit and climate controls
3) cut out the bottom part of the plastic cage that fits around the headunit and climate controls (so you can move the climate control unit down)
4)break the guide-pins off of the back of the climate control unit, and bolt the unit through the holes where the pins used to go into (this should lower it just enough to line up with the new dash)
5) build a cage/use radiostraps to put double-din in, it'll require a bit of adjusting to get it straight and centered
6) put the new bezel on
Did it in my 02 avalanche, pretty easy process