Cash back rebates are taxable
Most states will charge sales tax on your cash incentive rebates.
Trade-in value is taxable
In a handfull of states (looking at you CA, HI, KY, MD, MI, MT and VA) there is no sales tax reduction when you trade in a car. In other words, they tax the full purchase price of the car.
Check this LINK for more info on taxability of your trade-in.