Pillsbury makes a line of sugar-free cake mixes and prepared icings. Email them for a carrot cake adaptation.
But you know the whole cakes does NOT have to be sugar free except if you are delivering it to a diabetic convention... If you have to make one tier white or chocolate sugarfree from a mix, then they should be flexible.
Here are some links to baking with splenda:
Susan Reid is an experienced test baker whose judgment I trust.