I have rust on bottom of my radiator cap

It is more likley that is not rust. The coolant GM uses is called Dexcool, Dexcool has had a reputation of causing a sludge that appears to be rust in the cooling system. I could get into the chemical break down, but there isn't enough room to explain. The main cause to this, is lack of maint. over time the coolant will build a charge of electricity called electrolisys, this is what causes what you are more than likely seeing. The correct way to eliminate this (if it isn't too bad) is to go to your local repair shop, and ask if they offer the BG cooling system flush w/ additive. If they do, go ahead and have them do it. If it is pretty bad have repeated after about 6000 miles. You may also find more info on the dexcool and its effects here http://www.vnnforum.com/showthread.php?p=994256

The plug is on the driverside under the radiator flashing, I recommend you remove the radiator and have it flushed at a shop I also suggets you have the trasnmisson oil cooler cleand in the radiator as well and have it pressure tested and have it boiled out to get rid of bugs stuck in the radiator. it's a 1 hour job to remove it and $45.00 to have it done professionally. it's not worth risking burning the engine up by doing it yourself, when they pressure test it they may find leaking inner core coolers and wil have to be revored or soldered up so it does not leak. and the rust has to be neturalized with a chemical bath. then treated so it won't rust anymore and repainted, they will also tet the radiato r cap seal to see if it is bad and fix it as well and test the cap to see if it needs to be replaced. they will also test the oil cooler so you don't get wather into the transmisson oil tush damaging the trasmisson and or engine.