On the off chance that someone is still looking into a DIY way to flush our cooling systems, I'll explain my personal experience.
Firstly, there is no drain plug (at least on the 06 GTs with the 3.5 like mine).
Secondly, when it comes to flushing dexcool and replacing it with other coolants (I wanted Fords G-05 or whatever it was because supposedly it's top notch), they don't react to eachother very well, and can actually do more damage to our vehicles than good. So if you want to go that route, I'm going to say that you'll want to take your vehicle to whichever dealership has the coolant you want, and have them do it, because they'll have the equipment for an actual flush.
Now, my experience with flushing my coolant, which I did because I've been having some cooling issues lately (narrowed it down to bad wiring on the Cooling Fan 2 circuit, will be posting about fixing that in the how-to section later), involved a water hose, a large catch-tub (mine was about 2.5 foot long, a foot and a half wide, and 8-10 inches deep).
First, I jacked up the front end of the vehicle and put jackstands in place.
Next step was to remove the air intake piping (ease of access for next step).
now that that was removed, when you look down between the rad and the engine on the passenger side, there's the hose that connects the water pump to the radiator, position your catch-tub underneath this, under the car (this is why I jacked it up, no where NEAR enough room to fit the tub underneath the car on my driveway).
Using a pair of channel lock pliers, i pulled the clamp down the hose on the water pump side.
Next, i wiggled off the hose and left the coolant drain until there was no more coming out.
Re-attach the hose and put the clamp back in place.
Replace the intake piping.
On the 3.5 Litre engines you'll see a brass bleed-plug towards the front of the engine on the passenger side, connected to some black tubing. remove this (just the small plug, not the entire brass assembly).
now, fill your coolant reservoir with water and keep an eye on that brass fitting. when you see a stream of liquid coming out (solid stream, not sputtering) you can stop adding water. replace the brass plug. (this is where I used the water hose, I know, dirty me, not using distilled water in our tempermental cooling systems haha).
Turn the vehicle on and turn the a/c to max. your cooling system should kick on and mix the water with the remaining coolant-water mix.
after about 3-10 minutes it should be sufficiently mixed.
Now, repeat this process of removing the intake piping, removing the coolant hose, draining, reattaching the coolant hose, reattaching the intake piping, removing the brass plug, filling the coolant, cycling the coolant, until what you drain out during the rad hose removal is clear water.
for safety's sake (and maybe because I'm a little picky about being thorough) you can repeat the process one more time.
Now, you should have ~99.9% water in your cooling system, and you should be able half full.
Add whatever the hell you like for coolant (use that brass bleed plug when filling, as it lets all the air out of the cooling system (burp it the traditional way just for added peace of mind if you like), and enjoy your new coolant.
Now, as I said, I've read a fair bit about other coolants not agreeing with dexcool, so in no way do I take ANY responsibility if this buggers your system. Your choice of coolant is you preference, I only gave you my experience on how to flush the system. (hell, *I* put dex-cool back in to be on the safe side).
On another note, if you want to get the PERFECT 50/50 mix, keep track of how many time you drain the system and keep track of how MUCH liquid you've drained from the system.
Divide the amount by the number of times, and subtract that from the cooling system's capacity (Ask any GM technician and they should be able to tell you the capacity).
The amount you get will be the amount of water in your system, I believe it should be a bit less than half of the capacity (could be more, I wasn't picky enough to do this, it's just a thought for the next guy). from there, add whatever amount of water you need to fill the system to half capacity, and the rest coolant. Tada. 50/50 mix and a flushed (in a down-home, red-neck kinda way) system.
If you'll excuse me, I'm going to copy and paste this into it's own post in the How-To for everyone's benefit, as it's MUCH longer than I originally planned, and it should make it easier to find.
Take car everyone, and remember "Keep your stick on the ice"
In nomine Patris, et Filii, et Spiritus Sancti. Amen.