I have the Raxiom / American Muscle sequential tail lamp kit, so all three segments of each tail lamp are always lit (when the lights are on). I should have mentioned that earlier. These are the lamps I'm using:
http://www.vleds.com/bulb-type/brake-lights/3157/3157-red.html. They are 3157-style direct replacements.
As to your question of whether I've done any wiring modifications, the answer is yes. My car is extensively modified mechanically and electrically, which makes things much trickier to diagnose. Here are the electrical modifications I've made (that I can remember):
Sequential tail lamps (as previously mentioned)
Bosch intercooler pump tied into random "key-on hot" wire in the passenger's side corner of the engine bay (I know, I know, I should be using a relay, but it's been working great for 5 years)
Speedhut gauges wired into head lamp switch
8 gauge wire feeding twin Walbro 255s, grounded to grounding lug in trunk (this is why I'm suspicious of back-feeding)
However, I haven't made any wiring modifications since installing the LEDs, and everything was working great with the regular incandescent bulbs.
Does this shed more light on the situation? (haha)