We might need a little more information. I would start with who did the installation. If you had a dealer or hitch shop do the install then I would take it back and ask them to make it right. If you did the install I would be looking for a loose or disconnected wire. I've seen these units and from what I remember they really are plug and play from a connector standpoint.
Last but not least when you say the tail light isn't working correctly is it the tail light on your CR-V or the tail light on the trailer when connected to the CR-V.
56158 has a fuse
Fuse is it is hard to get at and hard to tell if the fuse is blown and it is behind a bolted in panel.
Drill a small hole or slot in the panel and tie a string or rope tie it to the fuse block so you can pull the fuse block out with out unbolting the panel.