How do I calculate special depreciation allowance on a car I bought several years ago for $20,000?

Bonus or Special Depreciation election must be made in the year that the equipment, machinery or car is placed in service.  So if the equipment, machinery or car was placed in service three years ago, that tax return and each tax return subsequent to that would have to be amended.

Depending upon the year that the equipment, machinery or car was placed in service, you may or may not be allowed to claim Bonus or Special Depreciation on an amended tax return at a later time.

IRS Publication 946 How to depreciate property, states:

How Can You Elect Not To Claim an Allowance?

When to make election

Generally, you must make the election on a timely filed tax return (including extensions) for the year in which you place the property in service.  However, if you timely filed your return for the year without making the election, you can still make the election by filing an amended return within 6 months of the due date of the original return (not including extensions). Attach the election statement to the amended return. On the amended return, write “Filed pursuant to section 301.9100-2.”

Revoking an election

Once you elect not to deduct a special depreciation allowance for a class of property, you cannot revoke the election without IRS consent. A request to revoke the election is a request for a letter ruling.