'92 Audi 100 S Rear Light (Brake?) Setup

Ok, so I just posted a question about the same car not going into park thing, which is perhaps a bit vague and more of a complicated issue than someone can answer. This one is easy though I think.

The dash is telling me a bulb is out. The thing is the following seems to work:

Headlights
Front and rear turn signals
Parking lights
Reverse lights
Center brake light
Outer right and left brake light.

There is however a bulb on the rear left that doesn't illuminate in any of the scenarios. Its right next to the reverse light inside the trunk lid. On the right side, there is no bulb, but upon inspecting it, the bulb housing appears completely different than it does on the left. There is no bulb or wiring for a bulb on the right passenger side. It looks totally different than the left side. It is capped off, I guess is how I would describe it. Again, this is the bulb directly next to the reverse bulb inside the trunk lid. I'm assuming this light is also for the brakes, since the reflective cover on the outside of the trunk is red, but like I said the outer bulbs (parking lights) get brighter when the brake pedal is depressed.

Maybe someone who has a similar model Audi 100 and knows what I'm talking about. I'm sure this is why the bulb warning is on, on the dash. I could replace the left (driver's side) bulb which doesn't light up, but it doesn't look burnt out and the thing I'm most confused about is why its not the same setup on the right side.