I worked as a new car salesman for half the year last year and received a 1099-MISC from FCA US for incentives payments. Do I check the "This was a manufacturer's incentive payment" box?
Yes, that is exactly what a manufacturer's incentive is. You should check that box.
JulieS, thank you for your help. On my 2018 return, I guess I wasn't paying attention and did not check this box for the same 1099-MISC from FCA US LLC. Is it worth amending my 2018 return? If so, does it raise the chances of an audit on the amended return or my 2019 return?
Did you pay self-employment (SE) tax on the amount last year?
If you did pay SE tax, you should amend last year's return. If you paid the tax you will have a Schedule 4 in your 2018 return with a number on line 57.
This question doesn't appear on your tax return, it is used to determine is whether or not you should pay SE tax on the amount on the 1099-MISC. A manufacturer's incentive is not subject to SE tax, just to regular income tax.
If you amend your 2018 and take off the SE tax, that may increase your chance of audit a little, but this is a very clear case and I don't think you should worry about that.