Cute idea, pretty well implemented. You are right - the seam along the kickpanel could be better. Any combination of blurring, smudging, cloning from the original, copy/pasting, etc would fix that. No right or wrong - just what works best for you.
One thought, did you thing of getting rid of the seam between the front and back doors? Make it a two-door instead of a four-door. Intuition says that that would make it look less "cartooney" if that is your intention.
As to the kickpanel - personally, I would copy/paste in a tiny piece from the original - and using a soft eraser brush I would get rid of the hard edges on the tiny piece so it looks more natural. But that's just me, as I said above, any number of paths would get you to the same end.
Rad