Under the tax law since 2018, you can't deduct vehicle expenses for a personally owned vehicle used for work if you are a W-2 worker. Your employer can't give you a tax-free truck allowance. They can either,
a. require you to prove your mileage and expenses and reimburse you tax-free for your proved expenses, or
b. they include the truck allowance in your taxable wages, reported on your W-2, and subject to tax withholding, social security and medicare taxes.
As such, there is no depreciation that you claimed or could have claimed for using the truck in your W-2 job, and you don't have to adjust your cost basis.
For the 1099 side job, you either claimed or could have claimed depreciation as a business expense. If you used the standard mileage rate, you claimed that on line 9 of schedule C. Part of the standard mileage rate includes depreciation, the amount is shown in publication 463.
https://www.irs.gov/publications/p463#en_US_2023_publink100034059
If you claimed actual car expenses, your depreciation was included on line 13 of schedule C.
You need to adjust your basis by the actual depreciation you claimed or could have claimed from the side job. Ignore the W-2 job because you were not entitled to depreciation for that job.
This answer has been verified for accuracy by an Intuit expert employee
