Hello! I really should put a write-up on the forums for everyone, i get these emails all the time!
anyways, its pretty easy. Take out the headunit and climate controls, cut the metal bracket in the back, move climate controls down, and build some sort of cage for the HU.
Okay more detailed:
First thing first, take your dash bezel off, and remove the headunit (need a metal coat hanger or a HU removal tool to push the pin-holes), remove the climate control unit (two bolts, four guide-pins, and unplugging a harness), and remove the onstar controls (they just snap out if you tug on em)
One all of that stuff is outof the way, you'll have plenty of room to work.
first thing's first, you need to cut the bracket that is behind the cd player, in order to make room for the depth of a double-DIN unit. I used a dremel to get mine off, then taped up the end of the bracket so the wires wouldnt get damaged. It looks like so:
With that out of the way, you'll have plenty of room for the depth of the unit.
You are also going to need to lower the climate control unit. A lot of people thing they need to cut that crap out of their dash to move this down, but you really dont. Its just a matter of removing the bottom of the plastic dash cage
This is what your dash will look like before you do anything to the climate controls (when a 03+dash bezel is put on):
underneath where the climate control unit is, there will be the bottom part of the plastic cage. Bust that out of there, so the climate control unit can be dropped a half inch or so. you can cut it out nicely if you want. I just used a pair of pliers and snapped out sections until the unit could slide down.
After thats modified, measure how far down you need to move the climate control unit to match up with the new dash bezel. Once you figure that out, its just a matter of bolting it up. What i did is i removed the four plastic pins on the climate control unit, and just used the bolt-holes to mount the unit back in. I also put some non-hardening clay behind the bolt spots to decrease vibrations:
You will have to to some moving around/sanding to get everything to line up perfectly, there may be some overlap. I had to do a wee bit of sanding/bending to get mine to fit right:
After you get that all lined up, you should have plenty of room to put the double-DIN in there, as well as plenty of room to adjust it around so you can get it nice and lined up.
Now that everything fits, you just have to get the double-din lined up with the dash. i just used MDF shims to make a cage around the sides/bottom of the unit. You'll need shims on the side, and a section to fit between the climate control unit and double-din unit (there is a little gap that goes about 2" back, i just used a section of MDF)
I was able to put 3/4" MDF shims on each side of the double-DIN unit, as well as a 3/4" shim between the double DIN and climate control unit, although others have said they could not do this. Mine fits perfectly this way. I also screwed the MDF shims into the plastic cage just for a little extra support. With the 3/4" shims, the unit was tight enough to the point where i didnt need to screw the unit in. It wedges in there perfectly. IF you would like to screw the units in, you can get L-brackets that are designed for headunits at various places, or headunit straps. I didnt need these though