[me and my metro]

Those are neat 4” round lights that I have not seen before. You can purchase 4” round LED backup lights. We use them in our trucks.
You will not need a trailer light adaptor now. You now have 6 circuits, I would use a trailer light connector with 5 on the outside and one in the center. This is still a popular connector. You may want to install two connectors so you can disconnect the factory lights when the tail is installed. Be aware that the factory flasher may be load sensitive and not blink correctly with just an LED load. There are LED rated flashers for most cars that do not care about the load, they just flash at a predetermined rate.