On the prompt side, it's very simple, and can probably be done in a variety of ways. How we did it is to prepare a prompt with multiple "user" messages. The first one gives the instruction

you are given a reference and three candidates, which one of the candidates do you think is a match to the reference? Only output its identifier or a code when none is found

Not exactly that but something along those lines.

Then one "user" message per car (reference + candidates) with image + text indicating the type (reference or candidate) and an identifier (can be as simple as the index for the candidates).