Deductions & credits

Hope you can help. Very difficult to “get in” this year. New Question: I gave him a car, purchased in 2001 in 2015. (Yes, we keep cars for  a long time). We replaced that car with 179000 miles on it in Dec 2019. How do I handle the depreciation? I never took it from 2001 - 2015 and I have all the mileage. Mileage fluctuated for all those years, sometimes zero. In 2015, the car was probably worth about $6000. We traded it in for $1500. It was always “sometimes” used for business, so it doesn’t fall into the “did you convert to personal use” But obviously the base value of 26,191 doesn’t work because he didn’t acquire it til 2015. The savings for taxes is quite different between ignoring this step and adding in the depreciation re: mileage. Just want to do it correctly.