I'm glad you asked, because it made me look.
It is IRS Form 8936. You can get it
HERE.I know, it says 2009 at the top but it appears to be the most current version.
You should download the forms that Brammo has on its website which show that the Enertia is qualified for the credit. Those forms are
HERE.Then,
Personal use part of credit. Enter the smaller of line 10 or line 13 here and on Form 1040, line 53, or Form 1040NR, line 49. Check box c on that line and enter “8936” in the space next to that box.
If line 13 is smaller than line 10, see instructions
How's that?