If you've used this vehicle in the past for business purposes, but have not claimed CCA on it, you will use the FMV of the vehicle the first year it was used for business purposes as your UCC.
You're not required to claim CCA, and if you don't, then the value carries forward until if and when you do claim CCA. So the FMV of the vehicle, when it was first used for business purposes, is what will have carried forward in your UCC balance until the current tax year.
If this is your first year of using the vehicle for business purposes, then you won't have a UCC amount. Instead, you'll need to include your vehicle as an "addition" and use the current Fair Market Value (FMV).