First, Dex did not eat your radiator or heater core. Dex is good for 5 years, or 100k miles, which ever comes first.

If the radiator was topped off with a coolant that is not compatible with dex, then there will be problems. It will sludge up, you will find a black sludge. Do you see this? What color is the coolant? Dex is a pink or orange color.

If it'* still under warranty, then they need to cover it. They will however try to find any excuse to get out doing the work. You may want to read over your paperwork.

I worked at a rad shop for years and we were finding that Dex Cool was turning very gritty even after 3 years. We had at least 10 cars where the impellar on the water pump had erroded away to just a stub.
Another thing that had become common was electrollisis, where the plastic tanked rads were not grounding and the end tubes on the rads were corroding and plugging and leaking. That is probably why the dealer wanted a ground strap put on your rad.
Your after market equipment may of voided the warranty by not being properly grounded.
Try using a volt meter and see if you get any volt reading in your coolant. Black to ground, remove rad cap and dip your positive lead from volt meter into coolant see if you get any reading.
We were finding high readings on vehicles that had aftermarket equipment, such as police cruisers and military vehicles. Check all your grounds and add some extras.

sounds like a good check. im interested to hear from a pro what would you replace the dexcool with?

I would still use Dex Cool but flush and fill every 2 maybe 3 years.
I have been in the GM plant in Oshawa and they add one of those coolant stop leak discs in each vehicle on the assembly line.
The reason = less warranty claims for minor leaks.

thats pretty messed up, maybe thats were that brown stuff comes from. people say air in the system and lack of maintenance but my 03 monte was less than two years old and had about 40k i think when my bottle started turning brown. i hadn't even took the rad cap off i just look at level on the over flow. ive run prestone extended life changed and flushed every couple of years since and still find some brown stuff on the cap when i do it

Just keep up with keeping it clean.

With a cleaning unit installed it becomes a "commercial vehicle" and thus the warranty can be declared null and void. Especially if this fact wasn't declared upon lease or purchase. Been there with work pickups in the past. Fine print is a killer.

Second, once I find something is not covered by a dealer, I will never let their idiotic profit driven help "play" with my car. One of the worse things about Dealers is they get away with soaking you for any possible way in the course of actually finding out whats really wrong with your car. This problem could even be a sticking actuator motor. Although the gunk is problematic here. An 08 or 09 with low miles and gunk found? It logically points to someone having added non Dex to the cars cooling system or oil is getting into the cooling stream somewhere in it'* path. It doesn't take much to do the trick. I don't trust shops and Dealers I don't know. So many these days are out to take you anyway they can. Dealer credibility? ya. That went out the window years ago. Especially since GM has made it so they have to document repairs so strickly to get paid, you would think your car problem was a Murder One case.

Could the coolant be mixed with regular tap water? if not distled water wasn't used it could have messed up the heater core.. But on the other hand it'* still not enough miles/time to have done so... anyway is the heater core leaking or something?