I'm using Turbo Tax, and it was not really obvious how to claim this - and it's a pretty decent credit, so it's worth digging.
According to the IRS, you can claim "A new plug-in conversion credit of 10% of the cost of converting any motor vehicle (new or used) to a qualified plug-in electric drive motor vehicle. The maximum credit is $4,000 per vehicle. This credit is claimed on Form 8910, Alternative Motor Vehicle Credit, and applies to property placed in service after February 17, 2009."
So, you need to go to the forms view in TurboTax and search for Form 8910. Fill in the worksheet (I put the complete itemization in just to have redundancy for the records) - say "yes" to item 2 ("Did you convert your motor vehicle to a qualified plug-in electric drive motor vehicle and place it in service after Feb 17 2009?"), fill in the date on line 2A, and the cost in line 2B, and it puts everything else where it should go. I did not attempt to manually deduct any of the expenses, so line 2C was zero.
According to the IRS, you can claim "A new plug-in conversion credit of 10% of the cost of converting any motor vehicle (new or used) to a qualified plug-in electric drive motor vehicle. The maximum credit is $4,000 per vehicle. This credit is claimed on Form 8910, Alternative Motor Vehicle Credit, and applies to property placed in service after February 17, 2009."
So, you need to go to the forms view in TurboTax and search for Form 8910. Fill in the worksheet (I put the complete itemization in just to have redundancy for the records) - say "yes" to item 2 ("Did you convert your motor vehicle to a qualified plug-in electric drive motor vehicle and place it in service after Feb 17 2009?"), fill in the date on line 2A, and the cost in line 2B, and it puts everything else where it should go. I did not attempt to manually deduct any of the expenses, so line 2C was zero.