It's not even that hard, the single contact on end of bulb MUST go in the single contact socket, the dual one goes in dual socket. You look inside the sockets to determine number of contacts there and same on any car on the planet. The bulbs also have different pins on the sides, they are stepped high/low on dual contact bulbs and in line on singles since the orientation does not matter. So, butchery job if they are transposed since bulb was forced into a socket the pins don't line up with, and how one tears up sockets.
LOOK AND THINK should cure any issues there. Wagon or no. You can't have a 'second filament' for the reverse since the reverse light physically is in a separate position with a clear lens.