The maximum would still be $18,000 in the first year, regardless if you 'maxed out' things with Section 179 or Bonus Depreciation.
No, Bonus shouldn't show up on Line 14 because a vehicle is "Listed Property". It should show up on Line 25 (page 2).
Yes, if you did use Section 179 instead of Bonus, you will want to amend 2018 to remove the Section 179 (and use Bonus, assuming you did not "elect out" of Bonus). The result should be the same on the Federal return, but your State return may change.
Although Section 179 and Bonus may be very similar, they can act very differently in combination with the Luxury Limits for vehicles.
If that is NOT the problem (you used Bonus in 2018, not 179), then I'm unsure why you are getting those results. Did the business percentage of the vehicle drop to 50% or less in 2019?
