For all those that have suffered getting water into their A8 tail lights and the water corroding and destroying everything inside.
Once the electronics start to corrode/rust there's really no other choice but to replace it with a new one. Quite costly if you ask me, even 1 tail light costs at the very least $200 from the junkyards or ebay (yes I checked).
I've had my 04 A8 for about a year and the tail-light on the trunk (passenger side) has always seem wet after it rains. I ignored it for a while thinking it was just condensation until the light gave out completely.
Here's the tail-light with water in it.
The voltage controller board for the LEDs has died, a lot of rust and corrosion. The board is actually made by Tyco. The 2 black wires coming out of the board are the + and - leads to the LEDs.
Here is the LED board. Surprisingly, most of the LEDs still work, only a couple are dead. I cut off the leads to the board and tested the LEDs with a 9v battery and they lit up.
So for the fix, basically the LEDs still work but will need switched power. I figured the easiest way would be to tap into the driver side tail-light and wire them together.
I soldered the + and - wires to a new wire and ran them out the rubber grommet.
The latter part of the night was spent taking the other tail-lamp out and splicing a wire to the same + and - leads on the LED board. I re-installed everything and now both sides light up. Be sure to use some extra silicone or other sealant to seal up the grommets and rubber gaskets on the tail-lights. That's how the water came in, the rubber gasket was old and brittle.
Also, I am fairly certain the LEDs on all the tail-lights *can* be un-soldered and replaced individually if they go out.