After more research I believe I have found the answer!
In Europe the E36's came wired with "rear fog lights". When activated the outer taillight would glow brighter (ala 2nd filament) so that drivers in fog could see you better from behind.
This option was not available in the US and is not wired up through the harness. However the socket and bulb were still dual filament.
Here are a couple of sites I found detailing this.
The second is a write up to re-enable them, I have no desire to do so, but anything is possible.