I don't know about the different rear light units, sorry. Loki has explained.
A couple of simple things though.
Re the bulb failure warning light, doe your car have a high level brake light? I have heard that has to be "factored in" by the bulb warning system. If it is original to the car, then if that bulb is not working, the system will detect that. If it is an accessory, the bulb warning system will detect an inbalance between left and right....that's how the bulb failure works. Oh and are all the brke light bulbs in definitely good condition, no filament weaknesses, and of matching quality?
Re water. Where is it coming from? Is water getting past the seal between the light unit and car body, or ..... ???
Good luck.