The deduction was to increase the amount of hybrids on the road. Hence, only for new vehicles. If you buy a used model, it doesn't add another hybrid to the mix. It's a way to influence growth. (my ECON 312 class is finally paying off).
But I do agree; this incentive should be open to both new and used.
Since you're in CO, maybe it would be worthwhile to sell the '04 HCH and buy a new Prius? Sheesh! What is it now, $6500 in credits/deductions for that car in your state?
