Here's the simplified facts.
*No* *matter* *what* *you* *do*, if you took the full SEC179 deduction on the old car in the year you purchased it or placed it in service, that means you deducted the entire cost of that car from your taxable income in the tax year you placed it in service. Therefore, your cost basis on that old car is "ZERO". That's because you have already deducted the full cost of the car in the tax year you purchased it, from your taxable income in that tax year. So if you sell that car for $1, that $1 is reportable and potentially taxable income.
So for a trade-in, that would make your trade-in value ZERO. If you donated it to a qualify charity, your qualified charitable donation amount is ZERO. If you junked it, it's junk value is ZERO. If you sold it for scrap, then whatever you sold it for is reportable/taxable income. If the car wasl stolen, it's value prior to the theft was zero, and after the theft was zero, and any insurance payout is reportable (and possibly taxable) income, regardless of what you did with that payout.
